Exposure Part 4: ISO

So far in this series we have seen how we can adjust exposure using aperture, which affects depth of field, ND filters, which can help us retain the depth of field we want, and shutter angle, which affects motion blur and flickering of certain light sources. In this final part we’ll look at ISO, perhaps the most misunderstood element of exposure, if indeed we can technically classify it as part of exposure at all!

 

What is ISO?

The acronym stands for International Organization for Standardization, the body which in 1974 combined the old ASA (American Standards Association) units of film speed with the German DIN standard. That’s why you’ll often hear the terms ISO and ASA used interchangeably.

Two different cameras filming the same scene with the same filters, aperture and shutter settings will not necessarily produce an image of equal brightness, because the ways that their electronics convert light into video signals are different. That is why we need ISO, which defines the relationship between the amount of light reaching the sensor (or film) and the brightness of the resulting image.

For example, a common ISO to shoot at today is 800. One way of defining ISO 800 is that it’s the setting required to correctly expose a key-light of 12 foot-candles with a lens set to T2.8 and a 180° shutter at 24fps (1/48th of a second).

If we double the ISO we double the effective sensitivity of the camera, or halve the amount of light it requires. So at ISO 1600 we would only need 6 foot-candles of light (all the other settings being the same), and at ISO 3200 we would need just 3 foot-candles. Conversely, at ISO 400 we would need about 25 foot-candles, or 50 at ISO 200.

 

A Flawed Analogy

Note that I said “effective” sensitivity. This is an important point. In the photochemical world, ISO indeed denotes the light sensitivity of the film stock. It is tempting to see digital ISO as representing the sensitivity of the sensor, and changing the ISO as analogous to loading a different film stock. But in reality the sensitivity of a digital sensor is fixed, and the ISO only determines the amount of gain applied to the sensor data before it is processed (which may happen in camera if you’re shooting linear or log, or in post if you’re shooting RAW).

So a better analogy is that altering the ISO is like altering how long the lab develops the exposed film negative for. This alters the film’s exposure index (EI), hence some digital cameras using the term EI in their menus instead of ISO or ASA.

We can take this analogy further. Film manufacturers specify a recommended development time, an arbitrary period designed to produce the optimal image. If you increase (push) or decrease (pull) the development time you will get a lighter or darker image respectively, but the quality of the image will be reduced in various ways. Similarly, digital camera manufacturers specify a native ISO, which is essentially the recommended amount of gain applied to the sensor data to produce what the manufacturer feels is the best image, and if you move away from that native ISO you’ll get a subjectively “lower quality” image.

Compare the graininess/smoothness of the blacks in these images from my 2017 tests. Click to enlarge.

The most obvious side effect of increasing the ISO is more noticeable noise in the image. It’s exactly the same as turning up the volume on an amplifier; you hear more hiss because the noise floor is being boosted along with the signal itself.

I remember the days of Mini-DV cameras, which instead of ISO had gain; my Canon XL1 had gain settings of -3dB, +6dB and +12dB. It was the exact same thing, just with a different name. What the XL1 called 0dB of gain was what we call the native ISO today.

 

ISO and Dynamic range

At this point we need to bring in the concept of dynamic range. Let’s take the Arri Alexa as an example. This camera has a dynamic range of 14 stops. At its native ISO of 800, those 14 stops of dynamic range are equally distributed above and below “correct” exposure (known as middle grey), so you can overexpose by up to seven stops, and underexpose by up to seven stops, without losing detail.

If you change the Alexa’s ISO, those limits of under- and overexposure still apply, but they’re shifted around middle grey. For example, at 400 ISO you have eight stops of detail below middle grey, but only six above it. This means that, assuming you adjust your iris, shutter or filters to compensate for the change in ISO, you can trade-off highlight detail for shadow detail, or vice versa.

Imagine underexposing a shot by one stop and bringing it back up in post. You increase the highlight detail, because you’re letting half the light through to the sensor, reducing the risk of clipped whites, but you also increase the noise when you bring it up in post. This is basically what you’re doing when you increase your ISO, except that if you’re recording in linear or log then the restoration of brightness and increase in gain happen within the camera, rather than in post with RAW.

Note the increased detail in the bulb at higher ISOs. Click to enlarge..

We can summarise all this as follows:

Doubling the ISO…

  • increases overall brightness by one stop, and
  • increases picture noise.

Then adjusting the exposure to compensate (e.g. closing the iris one stop)…

  • restores overall brightness to its original value,
  • gives you one more stop of detail in the highlights, and
  • gives you one less stop of detail in the shadows.

Alternatively, halving the ISO…

  • decreases overall brightness by one stop, and
  • decreases picture noise.

Then adjusting the exposure to compensate (e.g. opening the iris one stop)…

  • restores overall brightness to its original value,
  • gives you one less stop of detail in the highlights, and
  • gives you one more stop of detail in the shadows.

 

Conclusion

This brings me to the end of my exposure series. We’ve seen that choosing the “correct” exposure is a balancing act, taking into account not just the intended brightness of the image but also the desired depth of field, bokeh, lens flares, motion blur, flicker prevention, noise and dynamic range. I hope this series has helped you to make the best creative decisions on your next production.

See also: “6 Ways to Judge Exposure”

Exposure Part 4: ISO

Exposure Part 3: Shutter

In the first two parts of this series we saw how exposure can be controlled using the lens aperture – with side effects including changes to the depth of field – and neutral density (ND) filters. Today we will look at another means of exposure control: shutter angle.

 

The Physical Shutters of Film Cameras

As with aperture, an understanding of what’s going on under the hood is useful, and that begins with celluloid. Let’s imagine we’re shooting on film at 24fps, the most common frame rate. The film can’t move continuously through the gate (the opening behind the lens where the focused light strikes the film) or we would end up recording just a long vertical streak of light. The film must remain stationary long enough to expose an image, before being moved on by a distance of four perforations (the standard height of a 35mm film frame) so that the next frame can be exposed. Crucially, light must not hit the film while it is being moved, or vertical streaking will occur.

Joram van Hartingsveldt, CC BY-SA 3.0

This is where the shutter comes in. The shutter is a portion of a disc that spins in front of the gate. The standard shutter angle is 180°, meaning that the shutter is a semi-circle. We always describe shutter angles by the portion of the disc which is missing, so a 270° shutter (admitting 1.5x the light of a 180° shutter) is a quarter of a circle, and a 90° shutter (admitting half the light of a 180° shutter) is three-quarters.

The shutter spins continuously at the same speed as the frame rate – so at 24fps the shutter makes 24 revolutions per second. So with a 180° shutter, each 24th of a second is divided into two halves, i.e. 48ths of a second:

  • During one 48th of a second, the missing part of the shutter is over the gate, allowing the light to pass through and the stationary film to be exposed.
  • During the other 48th of a second, the shutter blocks the gate to prevent light hitting the film as it is advanced. The shutter has a mirrored surface so that light from the lens is reflected up the viewfinder, allowing the camera operator to see what they’re shooting.

 

Intervals vs. Angles

If you come from a stills or ENG background, you may be more used to talking about shutter intervals rather than angles. The two things are related as follows:

Frame rate x (360 ÷ shutter angle) = shutter interval denominator

For example, 24 x (360 ÷ 180) = 48 so a film running at 24fps, shot with a 180° shutter, shows us only a 48th of a second’s worth of light on each frame. This has been the standard frame rate and shutter angle in cinema since the introduction of sound in the late 1920s. The amount of motion blur captured in a 48th of a second is the amount that we as an audience have been trained to expect from motion pictures all our lives.

A greater (larger shutter angle, longer shutter interval) or lesser (smaller shutter angle, shorter shutter interval) amount of motion blur looks unusual to us and thus can be used to creative effect. Saving Private Ryan features one of the best-known examples of a small shutter angle in its D-day landing sequence, where the lack of motion blur creates a crisp, hyper-real effect that draws you into the horror of the battle. The effect has been endlessly copied since then, to the point that it now feels almost mandatory to shoot action scenes with a small shutter angle.

Large shutter angles are less common, but the extra motion blur can imply a drugged, fatigued or dream-like state.

In today’s digital environment, only the Arri Alexa Studio has a physical shutter. In other cameras, the sensor’s photo-sites are allowed to charge with light over a certain period of time – still referred to as the shutter interval, even though no actual shutter is involved. The same principles apply and the same 180° angle of the virtual shutter is standard. The camera will allow you to select a shutter angle/interval from a number of options, and on some models like the Canon C300 there is a menu setting to switch between displaying the shutter setting as an angle or an interval.

 

When to Change the Shutter Angle

Sometimes it is necessary to change the shutter angle to avoid flickering. Some luminous devices, such as TV screens and monitors, or HMI lighting not set to flicker-free mode, will appear to strobe, pulse or roll on camera. This is due to them turning on and off multiple times per second, in sync with the alternating current of the mains power supply, but not necessarily in sync with the shutter. For example, if you shoot a domestic fluorescent lamp in the UK, where the mains AC cycles at 50Hz, your 1/48th (180° at 24fps) shutter will be out of sync and the lamp will appear to throb or flicker on camera. The solution is to set the shutter to 172.8° (1/50th), which is indeed what most DPs do when shooting features in the UK. Round multiples of the AC frequency like 1/100th will also work.

You may notice that I have barely mentioned exposure so far in this article. This is because, unlike stills photographers, DPs rarely use the shutter as a means of adjusting exposure. An exception is that we may increase the shutter angle when the daylight is fading, to grab an extra shot. By doubling the shutter angle from 172.8° to 345.6° we double the light admitted, i.e. we gain one stop. As long as there isn’t any fast movement, the extra motion blur is likely to go unnoticed by the audience.

One of the hallmarks of amateur cinematography is that sunny scenes have no motion blur, due to the operator (or the camera’s auto mode) decreasing the shutter interval to avoid over-exposure. It is preferable to use ND filters to cut light on bright days, as covered in part two of this series.

For the best results, the 180° (or thereabouts) shutter angle should be retained when shooting slow motion as well. If your camera displays intervals rather than angles, ideally your interval denominator should be double the frame rate. So if you want to shoot at 50fps, set the shutter interval to 1/100th. For 100fps, set the shutter to 1/200th, and so on.

If you do need to change the shutter angle for creative or technical reasons, you will usually want to compensate with the aperture. If you halve the time the shutter is open for, you must double the area of the aperture to maintain the same exposure, and vice versa. For example, if your iris was set to T4 and you change the shutter from 180° to 90° you will need to stop up to T2.8. (Refer back to my article on aperture if you need to refresh your memory about T-stops.)

In the final part of this series we’ll get to grips with ISO.

Learn more about exposure in my online course, Cinematic Lighting. Until this Thursday (19/11/20) you can get it for the special price of £15.99 by using the voucher code INSTA90.

Exposure Part 3: Shutter

Exposure Part 1: Aperture

This is the first in a series of posts where I will look in detail at the four means of controlling the brightness of a digital video image: aperture, neutral density (ND) filters, shutter angle and ISO. It is not uncommon for newer cinematographers to have only a partial understanding of these topics, enough to get by in most situations; that was certainly the case with me for many years. The aim of this series is to give you an understanding of the underlying mechanics which will enable you to make more informed creative decisions.

You can change any one of the four factors, or any combination of them, to reach your desired level of exposure. However, most of them will also affect the image in other ways; for example, aperture affects depth of field. One of the key responsibilities of the director of photography is to use each of the four factors not just to create the ideal exposure, but to make appropriate use of these “side effects” as well.

 

f-stops and t-stops

The most common way of altering exposure is to adjust the aperture, a.k.a. the iris, sometimes described as changing “the stop”. Just like the pupil in our eyes, the aperture of a photographic lens is a (roughly) circular opening which can be expanded or contracted to permit more or less light through to the sensor.

You will have seen a series of numbers like this printed on the sides of lenses:

1      1.4      2      2.8      4      5.6      8      11      16      22     32

These are ratios – ratios of the lens’ focal length to its iris diameter. So a 50mm lens with a 25mm diameter iris is at f/2. Other lengths of lens would have different iris diameters at f/2 (e.g. 10mm diameter for a 20mm lens) but they would all produce an image of the same brightness. That’s why we use f-stops to talk about iris rather than diameters.

But why not label a lens 1, 2, 3, 4…? Why 1, 1.2, 2, 2.8…? These magic numbers are f-stops. A lens set to f/1.4 will let in twice as much light as (or “one stop more than”) a lens set to f/2, which in turn will let in twice as much as one set to f/2.8, and so on. Conversely, a lens set to f/2.8 will let in half as much light as (or “one stop less than”) a lens set to f/2, and so on. (Note that a number between any of these f-stops, e.g. f/1.8, is properly called an f-number, but not an f-stop.) These doublings or halvings – technically known as a base-2 logarithmic scale – are a fundamental concept in exposure, and mimic our eyes’ response to light.

If you think back to high-school maths and the πr² squared formula for calculating the area of a circle from its radius, the reason for the seemingly random series of numbers will start to become clear. Letting in twice as much light requires twice as much area for those light rays to fall on, and remember that the f-number is the ratio of the focal length to the iris diameter, so you can see how square roots are going to get involved and why f-stops aren’t just plain old round numbers.

If you’re shooting with a cine lens, rather than a stills lens, you’ll see the same series of numbers on the barrel, but here they are T-stops rather than f-stops. T-stops are f-stops adjusted to compensate for the light transmission efficiency. Two different lenses set to, say, f/2 will not necessarily produce equally bright images, because some percentage of light travelling through the elements will always be lost, and that percentage will vary depending on the quality of the glass and the number of elements. A lens with 100% light transmission would have the same f-number and T-number, but in practice the T-number will always be a little bigger than the f-number. For example, Cooke’s 15-40mm zoom is rated at a maximum aperture of T2 or f/1.84.

 

Fast and slow lenses

When buying or renting a lens, one of the first things you will want to know is its maximum aperture. Lenses are often described as being fast (larger maximum aperture, denoted by a smaller f- or T-number like T1.4) or slow (smaller maximum aperture, denoted by a bigger f- or T-number like T4). These terms come from the fact that the shutter speed would need to be faster or slower to capture the same amount of light… but more on that later in the series.

Faster lenses are generally more expensive, but that expense may well be outweighed by the savings made on lighting equipment. Let’s take a simple example, and imagine an interview lit by a 4-bank Kino Flo and exposed at T2.8. If our lens can open one stop wider (known as stopping up) to T2 then we double the amount of light reaching the sensor. We can therefore halve the level of light – by turning off two of the Kino Flo’s tubes or by renting a cheaper 2-bank unit in the first place. If we can stop up further, to T1.4, then we only need one Kino tube to achieve the same exposure.

 

Side effects

One of the first things that budding cinematographers learn is that wider apertures make for a smaller depth of field, i.e. the range of distances within which a subject will be in focus is smaller. In simple terms, the background of the image is blurrier when the depth of field is shallower.

It is often tempting to go for the shallowest possible depth of field, because it feels more cinematic and helps conceal shortcomings in the production design, but that is not the right look for every story. A DP will often choose a stop to shoot at based on the depth of field they desire. That choice of stop may affect the entire lighting budget; if you want to shoot at a very slow T14 like Douglas Slocombe did for the Indiana Jones trilogy, you’re going to need several trucks full of lights!

There is another side effect of adjusting the aperture which is less obvious. Lenses are manufactured to perform best in the middle of their iris range. If you open a lens up to its maximum aperture or close it down to its minimum, the image will soften a little. Therefore another advantage of faster lenses is the ability to get further away from their maximum aperture (and poorest image quality) with the same amount of light.

Finally it is worth noting that the appearance of bokeh (out of focus areas) and lens flares also changes with aperture. The Cooke S4 range, for example, renders out-of-focus highlights as circles when wide open, but as octagons when stopped down. With all lenses, the star pattern seen around bright light sources will be stronger when the aperture is smaller. You should shoot tests – like these I conducted in 2017 – if these image artefacts are a critical part of your film’s look.

Next time we’ll look at how we can use ND filters to control exposure without compromising our choice of stop.

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Exposure Part 1: Aperture

How is Dynamic Range Measured?

The high dynamic range of the ARRI Alexa Mini allowed me to retain all the sky detail in this shot from “Above the Clouds”.

Recently I’ve been pondering which camera to shoot an upcoming project on, so I consulted the ASC’s comparison chart. Amongst the many specs compared is dynamic range, and I noticed that the ARRI Alexa’s was given as 14+ stops, while the Blackmagic URSA’s is 15. Having used both cameras a fair bit, I can tell you that there’s no way in Hell that the Ursa has a higher dynamic range than the Alexa. So what’s going on here?

 

What is dynamic range?

To put it simply, dynamic range is the level of contrast that an imaging system can handle. To quote Alan Roberts, who we’ll come back to later:

This is normally calculated as the ratio of the exposure which just causes white clipping to the exposure level below which no details can be seen.

A photosite on a digital camera’s sensor outputs a voltage proportional to the amount of light hitting it, but at some point the voltage reaches a maximum, and no matter how much more light you add, it won’t change. At the other end of the scale, a photosite may receive so little light that it outputs no voltage, or at least nothing that’s discernible from the inherent electronic noise in the system. These upper and lower limits of brightness may be narrowed by image processing within the camera, with RAW recording usually retaining the full dynamic range, while linear Rec. 709 severely curtails it.

In photography and cinematography, we measure dynamic range in stops – doublings and halvings of light which I explain fully in this article. One stop is a ratio of 2:1, five stops are 32:1, thirteen stops are almost 10,000:1

It’s worth pausing here to point out the difference between dynamic range and latitude, a term which is sometimes regarded as synonymous, but it’s not. The latitude is a measure of how much the camera can be over- or under-exposed without losing any detail, and is dependent on both the dynamic range of the camera and the dynamic range of the scene. (A low-contrast scene will allow more latitude for incorrect exposure than a high-contrast scene.)

 

Problems of Measurement

Before digital cinema cameras were developed, video had a dynamic range of about seven stops. You could measure this relatively easily by shooting a greyscale chart and observing the waveform of the recorded image to see where the highlights levelled off and the shadows disappeared into the noise floor. With today’s dynamic ranges into double digits, simple charts are no longer practical, because you can’t manufacture white enough paper or black enough ink.

For his excellent video on dynamic range, Filmmaker IQ’s John Hess built a device fitted with a row of 1W LEDs, using layers of neutral density gel to make each one a stop darker than its neighbour. For the purposes of his demonstration, this works fine, but as Phil Rhodes points out on RedShark News, you start running into the issue of the dynamic range of the lens.

It may seem strange to think that a lens has dynamic range, and in the past when I’ve heard other DPs talk about certain glass being more or less contrasty, I admit that I haven’t thought much about what that means. What it means is flare, and not the good anamorphic streak kind, but the general veiling whereby a strong light shining into the lens will raise the overall brightness of the image as it bounces around the different elements. This lifts the shadows, producing a certain amount of milkiness. Even with high contrast lenses, ones which are less prone to veiling, the brightest light on your test device will cause some glare over the darkest one, when measuring the kind of dynamic range today’s cameras enjoy.

 

Manufacturer Measurements

Going back to my original query about the Alexa versus the URSA, let’s see exactly what the manufacturers say. ARRI specifically states that its sensor’s dynamic range is over 14 stops “as measured with the ARRI Dynamic Range Test Chart”. So what is this chart and how does it work? The official sales blurb runs thusly:

The ARRI DRTC-1 is a special test chart and analysis software for measurement of dynamic range and sensitivity of digital cameras. Through a unique stray light reduction concept this system is able to accurately measure up to 15.5 stops of dynamic range.

The “stray light reduction” is presumably to reduce the veiling mentioned earlier and provide more accurate results. This could be as simple as covering or turning off the brighter lights when measuring the dimmer ones.

I found a bit more information about the test chart in a 2011 camera shoot-out video, from that momentous time when digital was supplanting film as the cinematic acquisition format of choice. Rather than John Hess’s ND gel technique, the DRTC-1 opts for something else to regulate its light output, as ARRI’s Michael Bravin explains in the video:

There’s a piece of motion picture film behind it that’s checked with a densitometer, and what you do is you set the exposure for your camera, and where you lose detail in the vertical and horizontal lines is your clipping point, and where you lose detail because of noise in the shadow areas is your lowest exposure… and in between you end up finding the number of stops of dynamic range.

Blackmagic Design do not state how they measure the dynamic range of their cameras, but it may be a DSC Labs Xlya. This illuminated chart boasts a shutter system which “allows users to isolate and evaluate individual steps”, plus a “stepped xylophone shape” to minimise flare problems.

Art Adams, a cinema lens specialist at ARRI, and someone who’s frequently quoted in Blain Brown’s Cinematography: Theory & Practice, told Y.M. Cinema Magazine:

I used to do a lot of consulting with DSC Labs, who make camera test charts, so I own a 20-stop dynamic range chart (DSC Labs Xyla). This is what most manufacturers use to test dynamic range (although not ARRI, because our engineers don’t feel it’s precise enough) and I see what companies claim as usable stops. You can see that they are just barely above the noise floor.

 

Conclusions

Obviously these ARRI folks I keep quoting may be biased. I wanted to find an independent test that measures both Blackmagics and Alexas with the same conditions and methodology, but I couldn’t find one. There is plenty of anecdotal evidence that Alexas have a bigger dynamic range, in fact that’s widely accepted as fact, but quantifying the difference is harder. The most solid thing I could find is this, from a 2017 article about the Blackmagic Ursa Mini 4.6K (first generation):

The camera was measured at just over 14 stops of dynamic range in RAW 4:1 [and 13 stops in ProRes]. This is a good result, especially considering the price of the camera. To put this into perspective Alan measured the Canon C300 mkII at 15 stops of dynamic range. Both the URSA Mini 4.6 and C300 mkII are bettered by the ARRI Alexa and Amira, but then that comes as no surprise given their reputation and price.

The Alan mentioned is Alan Roberts, something of a legend when it comes to testing cameras. It is interesting to note that he is one of the key players behind the TLCI (Television Lighting Consistency Index), a mooted replacement for CRI (Colour Rendering Index). It’s interesting because this whole dynamic range business is starting to remind me of my investigation into CRI, and is leading me to a similar conclusion, that the numbers which the manufacturers give you are all but useless in real-world cinematography.

Whereas CRI at least has a standardised test, there’s no such thing for dynamic range. Therefore, until there is more transparency from manufacturers about how they measure it, I’d recommend ignoring their published values. As always when choosing a camera, shoot your own tests if at all possible. Even the most reliable numbers can’t tell you whether you’re going to like a camera’s look or not, or whether it’s right for the story you want to tell.

When tests aren’t possible, and I know that’s often the case in low-budget land, at least try to find an independent comparison. I’ll leave you with this video from the Slanted Lens, which compares the URSA Mini Pro G2 with the ARRI Amira (which uses the same Alev III sensor as the Alexa). They don’t measure the dynamic range, but you can at least see the images side by side, and in the end it’s the images that matter, not the numbers.

How is Dynamic Range Measured?

Making an Analogue Print

This is the latest in my series about analogue photography. Previously, I’ve covered the science behind film capture, and how to develop your own black-and-white film. Now we’ll proceed to the next step: taking your negative and producing a print from it. Along the way we’ll discover the analogue origins of Photoshop’s dodge and burn tools.

 

Contact printing

35mm contact sheet

To briefly summarise my earlier posts, we’ve seen that photographic emulsion – with the exception of colour slide film – turns black when exposed to light, and remains transparent when not. This is how we end up with a negative, in which dark areas correspond to the highlights in the scene, and light areas correspond with the shadows.

The simplest way to make a positive print from a negative is contact-printing, so called because the negative is placed in direct contact with the photographic printing paper. This is typically done in a spring-loaded contact printing frame, the top of which is made of glass. You shine light through the glass, usually from an enlarger – see below – for a measured period of time, determined by trial and error. Where the negative is dark (highlights) the light can’t get through, and the photographic emulsion on the paper remains transparent, allowing the white paper base to show through. Where the negative is transparent (shadows) the light passes through, and the emulsion – once developed and fixed in the same way as the original film – turns black. Thus a positive image is produced.

Normally you would contact-print multiple strips of negative at the same time, perhaps an entire roll of film’s worth, if your paper is large enough to fit them all. Then you can examine them through a loupe to decide which ones are worth enlarging. You have probably seen contact sheets, complete with circled images, stars and arrows indicating which frames the photographer or picture editor likes, where they might crop it, and which areas need doctoring. In fact, contact sheets are so aesthetically pleasing that it’s not uncommon these days for graphic designers to create fake digital ones.

The correct exposure time for a contact print can be found by exposing the whole sheet for, say, ten seconds, then covering a third of it with a piece of card, exposing it for another ten seconds, then covering that same third plus another third and exposing it for ten seconds more. Once developed, you can decide which exposure you like best, or try another set of timings.

120 contact sheet

 

Making an enlargement

Contact prints are all well and good, but they’re always the same size as the camera negative, which usually isn’t big enough for a finished product, especially with 35mm. This is where an enlarger comes in.

An enlarger is essentially a projector mounted on a stand. You place the negative of your chosen image into a drawer called the negative carrier. Above this is a bulb, and below it is a lens. When the bulb is turned on, light shines through the negative, and the lens focuses the image (upside-down of course) onto the paper below. By adjusting the height of the enlarger’s stand, you can alter the size of the projected image.

Just like a camera lens, an enlarger’s lens has adjustable focus and aperture. You can scrutinise the projected image using a loupe; if you can see the grain of the film, you know that the image is sharply focused.

The aperture is marked in f-stops as you would expect, and just like when shooting, you can trade off the iris size against the exposure time. For example, a print exposed for 30 seconds at f/8 will have the same brightness as one exposed for 15 seconds at f/5.6. (Opening from f/8 to f/5.6 doubles the light, or increases exposure by one stop, while halving the time cuts the light back to its original value.)

 

Dodging and burning

As with contact-printing, the optimum exposure for an enlargement can be found by test-printing strips for different lengths of time. This brings us to dodging and burning, which are respectively methods of decreasing or increasing the exposure time of specific parts of the image.

Remember that the printing paper starts off bright white, and turns black with exposure, so to brighten part of the image you need to reduce its exposure. This can be achieved by placing anything opaque between the projector lens and the paper for part of the exposure time. Typically a circle of cardboard on a piece of wire is used; this is known as a dodger. That’s the “lollipop” you see in the Photoshop icon. It’s important to keep the dodger moving during the exposure, otherwise you’ll end up with a sharply-defined bright area (not to mention a visible line where the wire handle was) rather than something subtle.

I dodged the robin in this image, to help him stand out.

Let me just say that dodging is a joyful thing to do. It’s such a primitive-looking tool, but you feel like a child with a magic wand when you’re using it, and it can improve an image no end. It’s common practice today for digital colourists to power-window a face and increase its luminance to draw the eye to it; photographers have been doing this for decades and decades.

Burning is of couse the opposite of dodging, i.e. increasing the exposure time of part of the picture to make it darker. One common application is to bring back detail in a bright sky. To do this you would first of all expose the entire image in such a way that the land will look good. Then, before developing, you would use a piece of card to cover the land, and expose the sky for maybe five or ten seconds more. Again, you would keep the card in constant motion to blend the edges of the effect.

To burn a smaller area, you would cut a hole in a piece of card, or simply form your hands into a rough hole, as depicted in the Photoshop icon.

 

Requirements of a darkroom

The crucial thing which I haven’t yet mentioned is that all of the above needs to take place in near-darkness. Black-and-white photographic paper is less sensitive to the red end of the spectrum, so a dim red lamp known as a safe-light can be used to see what you’re doing. Anything brighter – even your phone’s screen – will fog your photographic paper as soon as you take it out of its lightproof box.

Once your print is exposed, you need to agitate it in a tray of diluted developer for a couple of minutes, then dip it in a tray of water, then place it in a tray of diluted fixer. Only then can you turn on the main lights, but you must still fix the image for five minutes, then leave it in running water for ten minutes before drying it. (This all assumes you’re using resin-coated paper.)

Because you need an enlarger, which is fairly bulky, and space for the trays of chemicals, and running water, all in a room that is one hundred per cent lightproof, printing is a difficult thing to do at home. Fortunately there are a number of darkrooms available for hire around the country, so why not search for a local one and give analogue printing a go?

Some enlargements from 35mm on 8×10″ paper

 

Making an Analogue Print

How Analogue Photography Can Make You a Better Cinematographer

With many of us looking for new hobbies to see us through the zombie apocalypse Covid-19 lockdown, analogue photography may be the perfect one for an out-of-work DP. While few of us may get to experience the magic and discipline of shooting motion picture film, stills film is accessible to all. With a range of stocks on the market, bargain second-hand cameras on eBay, seemingly no end of vintage glass, and even home starter kits for processing your own images, there’s nothing to stop you giving it a go.

Since taking them up again In 2018, I’ve found that 35mm and 120 photography have had a positive impact on my digital cinematography. Here are five ways in which I think celluloid photography can help you too sharpen your filmmaking skills.

 

1. Thinking before you click

When you only have 36 shots on your roll and that roll cost you money, you suddenly have a different attitude to clicking the shutter. Is this image worthy of a place amongst those 36? If you’re shooting medium or large-format then the effect is multiplied. In fact, given that we all carry phone cameras with us everywhere we go, there has to be a pretty compelling reason to lug an SLR or view camera around. That’s bound to raise your game, making you think longer and harder about composition and content, to make every frame of celluloid a minor work of art.

 

2. Judging exposure

I know a gaffer who can step outside and tell you what f-stop the light is, using only his naked eye. This is largely because he is a keen analogue photographer. You can expose film by relying on your camera’s built-in TTL (through the lens) meter, but since you can’t see the results until the film is processed, analogue photographers tend to use other methods as well, or instead, to ensure a well-exposed negative. Rules like “Sunny Sixteen” (on a sunny day, set the aperture to f/16 and the shutter speed reciprocal to match the ISO, e.g. 1/200th of a second at ISO 200) and the use of handheld incident meters make you more aware of the light levels around you. A DP with this experience can get their lighting right more quickly.

 

3. Pre-visualising results

We digital DPs can fall into the habit of not looking at things with our eyes, always going straight to the viewfinder or the monitor to judge how things look. Since the optical viewfinder of an analogue camera tells you little more than the framing, you tend to spend less time looking through the camera and more using your eye and your mind to visualise how the image will look. This is especially true when it comes to white balance, exposure and the distribution of tones across a finished print, none of which are revealed by an analogue viewfinder. Exercising your mind like this gives you better intuition and increases your ability to plan a shoot, through storyboarding, for example.

 

4. Grading

If you take your analogue ethic through to post production by processing and printing your own photographs, there is even more to learn. Although detailed manipulation of motion pictures in post is relatively new, people have been doctoring still photos pretty much since the birth of the medium in the mid-19th century. Discovering the low-tech origins of Photoshop’s dodge and burn tools to adjust highlights and shadows is a pure joy, like waving a magic wand over your prints. More importantly, although the printing process is quick, it’s not instantaneous like Resolve or Baselight, so you do need to look carefully at your print, visualise the changes you’d like to make, and then execute them. As a DP, this makes you more critical of your own work and as a colourist, it enables you to work more efficiently by quickly identifying how a shot can be improved.

 

5. Understanding

Finally, working with the medium which digital was designed to imitate gives you a better understanding of that imitation. It was only when I learnt about push- and pull-processing – varying the development time of a film to alter the brightness of the final image – that my understanding of digital ISO really clicked. Indeed, some argue that electronic cameras don’t really have ISO, that it’s just a simulation to help users from an analogue background to understand what’s going on. If all you’ve ever used is the simulation (digital), then you’re unlikely to grasp the concepts in the same way that you would if you’ve tried the original (analogue).

How Analogue Photography Can Make You a Better Cinematographer

How to Make a Zoetrope for 35mm Contact Prints

Are you an analogue photographer looking for a different way to present your images? Have you ever thought about shooting a sequence of stills and reanimating them in a zoetrope, an optical device from the Victorian era that pre-figured cinema? That is exactly what I decided to do as a project to occupy myself during the zombie apocalypse Covid-19 lockdown. Contact prints are aesthetically pleasing in themselves, and I wanted to tap into the history of the zoetrope by creating a movie-like continuous filmstrip of sequential images and bringing them to life.

In the first part of my blog about this project, I covered the background and setting up a time-lapse of my cherry tree as content for the device. This weekend I shot the final image of the time-lapse, the last of the blossom having dropped. No-one stole my camera while it sat in my front garden for three weeks, and I was blessed with consistently sunny weather until the very last few days, when I was forced to adjust the exposure time to give me one or two extra stops. I’ll be interested to see how the images have come out, once I can get into the darkroom.

Meanwhile, I’ve been constructing the zoetrope itself, following this excellent article on Reframing Photography. Based on this, I’ve put together my own instructions specifically for making a device that holds 18 frames of contact-printed 35mm film. I chose a frame count of 18 for a few reasons:

  1. The resultant diameter, 220mm, seemed like a comfortable size, similar to a table lamp.
  2. Two image series of 18 frames fit neatly onto a 36 exposure film.
  3. Negatives are commonly cut into strips of six frames for storage and contact-printing, so a number divisible by six makes constructing the image loop a little more convenient.

 

You Will Need

  • Contact sheet containing 18 sequential 35mm images across three rows
  • A1 sheet of 300gsm card, ideally black
  • PVA glue
  • Ruler (the longer the better)
  • Set square
  • Compass
  • Pencil & eraser
  • Scissors
  • Craft knife or stanley knife
  • Paper clips or clothes pegs for clamping while glue dries
  • Rotating stand like a lazy susan or record player

 

Making the image loop

First, cut out the three rows of contact prints, leaving a bit of blank paper at one end of each row for overlap. Now glue them together into one long strip of 18 sequential images. The strip should measure 684mm plus overlap, because a 35mm negative or contact print measures 38mm in width including the border on one side: 38×18=684.

Glue the strip together into a loop with the images on the inside. This loop should have a diameter of 218mm. Note that we must make our zoetrope’s drum to a slightly bigger diameter, or the image loop won’t fit inside it. We’ll use our image loop to check the size of the drum; that’s why we’ve made it first. (If you don’t have your images ready yet, use an old contact sheet – as I did – or any strip of paper or light card of the correct size, 35mmx684mm.)

 

Making the side wall

Cut a strip of the black card measuring 723x90mm. This will be the side wall of your drum. Wrap this strip around your image loop, as tightly as you can without distorting the circular shape of the image loop. Mark where the card strip overlaps itself to find the circumference of the drum, which will be slightly bigger than the 684mm circumference of the image loop. In my case the drum circumference was 688mm – as illustrated in the diagram above. (You can click on it to enlarge it.)

Now we can measure and cut out the slots, one per image. Reframing Photography recommends a 1/8″ width, and initially I went with this, rounding it to 3mm. As with making a pinhole, a smaller slot means a sharper but darker image, while a bigger slot means a brighter but blurrier one. Once my zoetrope was complete, I felt that there was too much motion blur, so I retrofitted it with 1mm slots.

Let’s stick with 3x35mm (the same height as the images) for our slot size. How far apart should the slots be? They need to be evenly spaced around the circumference, so in my case 688÷18=38.2mm, i.e. a gap of 35.2mm between each slot and then 3mm for the slot itself. If your drum circumference is different to mine, you’ll have to do your own maths to work out the spacing.

(It was impossible to measure 38.2mm accurately, but I made a spreadsheet to give me values for the cumulative slot positions to the nearest millimetre: 38, 76, 115, 153, 191, 229, 268, 306, 344, 382, 420, 459, 497, 535, 573, 612, 650 and 688.)

Mark out your 18 slots, positioning them 15mm from the top of the side wall and 40mm from the bottom, then cut them out carefully using a knife and a ruler.

Now you can glue your side wall into a loop, using paper clips or clothes peg to hold it while the glue dries. I recommend double-checking your image loop fits inside beforehand. (Do not glue your image loop into the drum; this way you can swap it out for another image series whenever you like.)

 

Making the connector

The connector, as the name suggests, will connect the side wall to the base of the drum. (When I made a prototype, I tried skipping this stage, simply building the connecting teeth into the side wall, but this made it much harder to keep the drum a neat circle.)

Go back to your black card and cut another strip measuring 725x60mm. Score it all the way along the middle (i.e. 30mm from the edge) so that it can be folded in two, long-ways. Now cut triangular teeth into one half of the strip. Each triangle should have a 30mm base along the scored line.

As with the side wall, you should check the circumference of the connector to ensure that it will fit around the side wall and image loop, and adjust it if necessary. My connector’s circumference, as shown on the diagram above, was 690mm.

Glue the strip into a loop, clamping it with clips or pegs while it dries. Again, it doesn’t hurt to double-check that it still fits around the side wall first.

 

Making the base

Use a compass to draw a circle of 220mm in diameter on your remaining card, and cut it out. (If your connector is signficantly different in circumference to mine, divide that circumference by pi [3.14] to find the diameter that will work for you.)

Now you can glue the connector to the base. I suggest starting with a single tooth, putting a bottle of water or something heavy on it to keep it in place while it dries, then do the tooth directly opposite. Once that’s dry, do the ones at 90° and so on. This way you should prevent distortions creeping into the shape of the circle as you go around.

When that’s all dry, apply glue all around the inside of the upright section of the connector. Squish your side wall into a kidney bean shape to fit it inside the connector, then allow it to expand to its usual shape. If you have made it a tight enough fit, it will naturally press against the glue and the connector.

 

Making it Spin

The critical part of your zoetrope, the drum, is now complete. But to animate the images, you need to make it spin. There are a few ways you can do this:

  • Mount it on an old record player, making a hole in the centre of the base for the centre spindle.
  • Mount it on a rotating cake decoration stand or lazy susan.
  • Make your own custom stand.

I chose the latter, ordering some plywood discs cut to size, an unfinished candlestick and a lazy susan bearing, then assembling and varnishing them before gluing my drum to the top.

How to Make a Zoetrope for 35mm Contact Prints

Shooting a Time-lapse for a Zoetrope

Two years ago I made Stasis, a series of photographs that explored the confluence of time, space and light. Ever since then I’ve been meaning to follow it up with another photography project along similar lines, but haven’t got around to it. Well, with Covid-19 there’s not much excuse for not getting around to things any more.

Example of a zoetrope

So I’ve decided to make a zoetrope – a Victorian optical device which produces animation inside a spinning drum. The user looks through slits in the side of the drum to one of a series of images around the inside. When the drum is set spinning – usually by hand – the images appear to become one single moving picture. The slits passing rapidly through the user’s vision serve the same purpose as a shutter in a film projector, intermittently blanking out the image so that the persistence of vision effect kicks in.

Typically zoetropes contain drawn images, but they have been known to contain photographed images too. Eadward Muybridge, the father of cinema, reanimated some of his groundbreaking image series using zoetropes (though he favoured his proprietary zoopraxiscope) in the late nineteenth century. The device is thus rich with history and a direct antecedent of all movie projectors and the myriad devices capable of displaying moving images today.

This history, its relevance to my profession, and the looping nature of the animation all struck a chord with me. Stasis was to some extent about history repeating, so a zoetrope project seemed like it would sit well alongside it. Here though, history would repeat on a very small scale. Such a time loop, in which nothing can ever progress, feels very relevant under Covid-19 lockdown!

With that in mind, I decided that the first sequence I would shoot for the zoetrope would be a time-lapse of the cherry tree outside my window.  I chose a camera position at the opposite end of the garden, looking back at my window and front door – my lockdown “prison” – through the branches of the tree. (The tree was just about to start blooming.)

The plan is to shoot one exposure every day for at least the next 18 days, maybe more if necessary to capture the full life of the blossom. Ideally I want to record the blossom falling so that my sequence will loop neatly, although the emergence of leaves may interfere with that.

To make the whole thing a little more fun and primitive, I decided to shoot using the pinhole I made a couple of years ago. Since I plan to mount contact prints inside the zoetrope rather than enlargements, that’ll mean I’ve created and exhibited a motion picture without ever once putting the image through a lens.

I’m shooting on Ilford HP5+, a black-and-white stock with a published ISO of 400. My girlfriend bought me five roles for Christmas, which means I can potentially make ten 18-frame zoetrope inserts. I won’t be able to develop or print any of them until the lockdown ends, but that’s okay.

My first image was shot last Wednesday, a sunny day. The Sunny 16 rule tells me that at f/16 on a sunny day, my exposure should be equal to my ISO, i.e. 1/400th of a second for ISO 400. My pinhole has an aperture of f/365, which I calculated when I made it, so it’s about nine stops slower than f/16. Therefore I need to multiply that 1/400th of a second exposure time by two to the power of nine, which is 1.28 – call it one second for simplicity. ( I used my Sekonic incidence/reflectance meter to check the exposure, because it’s always wise to be sure when you haven’t got the fall-back of a digital monitor.)

One second is the longest exposure my Pentax P30t can shoot without switching to Bulb mode and timing it manually. It’s also about the longest exposure that HP5+ can do without the dreaded reciprocity failure kicking in. So all round, one second was a good exposure time to aim for.

The camera is facing roughly south, meaning that the tree is backlit and the wall of the house (which fills the background) is in shadow. This should make the tree stand out nicely. Every day may not be as sunny as today, so the light will inevitably change from frame to frame of the animation. I figured that maintaining a consistent exposure on the background wall would make the changes less jarring than trying to keep the tree’s exposure consistent.

I’ve been taking spot readings every day, and keeping the wall three-and-a-half stops under key, while the blossoms are about one stop over. I may well push the film – i.e. give it extra development time – if I end up with a lot of cloudy days where the blossoms are under key, but so far I’ve managed to catch the sun every time.

All this exposure stuff is great practice for the day when I finally get to shoot real motion picture film, should that day ever come, and it’s pretty useful for digital cinematography too.

Meanwhile, I’ve also made a rough prototype of the zoetrope itself, but more on that in a future post. Watch this space.

Shooting a Time-lapse for a Zoetrope

Pinhole Results

In my last couple of posts I described making and shooting with a pinhole attachment for my 35mm Pentax P30t SLR. Well, the scans are now back from the lab and I’m very pleased with them. They were shot on Fujifilm Superia Xtra 400.

As suspected, the 0.7mm pinhole was far too big, and the results are super-blurry:

See how contemptuous Spike is of this image. Or maybe that’s just Resting Cat Face.

The 0.125mm hole produced much better results, as you can see below. My f/stop calculations (f/365) seem to have been pretty close to the mark, although, as is often the case with film, the occasions where I gave it an extra stop of exposure produced even richer images. Exposure times for these varied between 2 and 16 seconds. Click to see them at higher resolution.

I love the ethereal, haunting quality of all these pictures, which recalls the fragility of Victorian photographs. It’s given me several ideas for new photography projects…

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Pinhole Results

Adventures with a Pinhole

Last week I discussed making a pinhole for my Pentax 35mm SLR. Since then I’ve made a second pinhole and shot a roll of Fujifilm Superia X-tra 400 with them. Although I haven’t had the film processed yet, so the quality of the images is still a mystery, I’ve found shooting with a pinhole to be a really useful exercise.

My Pentax P30T fitted with a 0.125mm pinhole attachment

 

A Smaller Pinhole

Soon after my previous post, I went out into the back garden and took ten exposures of the pond and the neighbour’s cat with the 0.7mm pinhole. By that point I had decided that the hole was almost certainly too big. As I noted last week, Mr Pinhole gives an optimal diameter of 0.284mm for my camera. Besides that, the (incredibly dark) images in my viewfinder were very blurry, a sign that the hole needed to be smaller.

Scans of my two pinholes

So I peeled the piece of black wrap with the 0.7mm pinhole off my drilled body cap and replaced it with another hole measuring about 0.125mm. I had actually made this smaller hole first but rejected it because absolutely nothing was visible through the viewfinder, except for a bit of a blur in the centre. But now I came to accept that I would have to shoot blind if I wanted my images to be anything approaching sharp.

The 0.125mm(ish) pinhole magnified in Photoshop

I had made the 0.125mm hole by tapping the black wrap with only the very tip of the needle, rather than pushing it fully through. Prior to taping it into the body cap, I scanned it at high resolution and measured it using Photoshop. This revealed that it’s a very irregular shape, which probably means the images will still be pretty soft. Unfortunately I couldn’t see a way of getting it any more circular; sanding didn’t seem to help.

Again I found the f-stop of the pinhole by dividing the flange focal distance (45.65mm) by the hole diameter, the result being about f/365. My incident-light meter only goes up to f/90, so I needed to figure out how many stops away from f/365 that is. I’m used to working in the f/1.4-f/22 range, so I wasn’t familiar with how the stop series progresses above f/90. Turns out that you can just multiply by 1.4 to roughly find the next stop up, so after f/90 it’s 128, then 180, then 256, then 358, pretty close to my f/365 pinhole. So whatever reading my meter gave me for f/90, I knew that I would need to add 4 stops of exposure, i.e. multiply the shutter interval by 16. (Stops are a base 2 logarithmic scale. See my article on f-stops, T-stops and ND filters for more info.)

 

The Freedom of Pinhole Shooting

I’ve just spent a pleasant hour or so in the garden shooting the remaining 26 exposures on my roll with the new 0.125mm pinhole. Regardless of how the photos come out, I found it a fun and fascinating exercise.

Knowing that the images would be soft made me concentrate on colour and form far more than I normally would. Not being able to frame using the viewfinder forced me to visualise the composition mentally. And as someone who finds traditional SLRs very tricky to focus, it was incredibly freeing not to have to worry about that, not to have to squint through the viewfinder at all, but just plonk the camera down where it looked right and squeeze the shutter.

Of course, before squeezing the shutter I needed to take incident-light readings, because the TTL (through the lens) meter was doing nothing but flash “underexposed” at me. Being able to rely solely on an incident meter to judge exposure is a very useful skill for a DP, so this was great practice. I’ve been reading a lot about Ansel Adams and the Zone System lately, and although this requires a spot reflectance meter to be implemented properly, I tried to follow Adams’ philosophy, visualising how I wanted the subject’s tones to correspond to the eventual print tones. (Expect an article about the Zone System in the not-too-distant future!)

 

D.I.Y. pinhole Camera

On Tuesday night I went along to a meeting of Cambridge Darkroom, the local camera club. By coincidence, this month’s subject was pinhole cameras. Using online plans, Rich Etteridge had made up kits for us to construct our own complete pinhole cameras in groups. I teamed up with a philosophy student called Tim, and we glued a contraption together in the finest Blue Peter style. The actual pinholes were made in metal squares cut from Foster’s cans, which are apparently something Rich has in abundance.

DIY pinhole camera

I have to be honest though: I’m quite scared of trying to use it. Look at those dowels. Can I really see any outcome of attempting to load this camera other than a heap of fogged film on the floor? No. I think I’ll stick with my actual professionally-made camera body for now. If the pinhole photos I took with that come out alright, then maaaaaaybe I’ll consider lowering the tech level further and trying out my Blue Peter camera. Either way, big thanks to Rich for taking all that time to produce the kits and talk us through the construction.

Watch this space to find out how my pinhole images come out.

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Adventures with a Pinhole