Creating “Stasis”

Stasis is a personal photography project about time and light. You can view all the images here, and in this post I’ll take you through the technical and creative process of making them.

I got into cinematography directly through a love of movies and filmmaking, rather than from a fine art background. To plug this gap, over the past few of years I’ve been trying to give myself an education in art by going to galleries, and reading art and photography books. I’ve previously written about how JMW Turner’s work captured my imagination, but another artist whose work stood out to me was Gerrit (a.k.a. Gerard) Dou. Whereas most of the Dutch 17th century masters painted daylight scenes, Dou often portrayed people lit by only a single candle.

“A Girl Watering Plants” by Gerrit Dou

At around the same time as I discovered Dou, I researched and wrote a blog post about Barry Lyndon‘s groundbreaking candlelit scenes. This got me fascinated by the idea that you can correctly expose an image without once looking at a light meter or digital monitor, because tables exist giving the appropriate stop, shutter and ISO for any given light level… as measured in foot-candles. (One foot-candle is the amount of light received from a standard candle that is one foot away.)

So when I bought a 35mm SLR (a Pentax P30T) last autumn, my first thought was to recreate some of Dou’s scenes. It would be primarily an exercise in exposure discipline, training me to judge light levels and fall-off without recourse to false colours, histograms or any of the other tools available to a modern DP.

I conducted tests with Kate Madison, who had also agreed to furnish period props and costumes from the large collection which she had built up while making Born of Hope and Ren: The Girl with the Mark. Both the tests and the final images were captured on Fujifilm Superia X-tra 400. Ideally I would have tested multiple stocks, but I must confess that the costs of buying and processing several rolls were off-putting. I’d previously shot some basic latitude tests with Superia, so I had some confidence about what it could and couldn’t do. (It can be over-exposed at least five stops and still look good, but more than a stop under and it falls apart.) I therefore confined myself to experimenting with candle-to-subject distances, exposure times and filtration.

The tests showed that the concept was going to work, and also confirmed that I would need to use an 80B filter to cool the “white balance” of the film from its native daylight to tungsten (3400K). (As far as I can tell, tungsten-balanced stills film is no longer on the market.) Candlelight has a colour temperature of about 1800K, so it still reads as orange through an 80B, but without the filter it’s an ugly red.

Meanwhile, the concept had developed beyond simply recreating Gerrit Dou’s scenes. I decided to add a second character, contrasting the historical man lit only by his candle with a modern girl lit only by her phone. Flames have a hypnotic power, tapping into our ancient attraction to light, and today’s smartphones have a similarly powerful draw.

The candlelight was 1600K warmer than the filtered film, so I used an app called Colour Temp to set my iPhone to 5000K, making it 1600K cooler than the film; the phone would therefore look as blue as the candle looked orange. (Unfortunately my phone died quickly and I had trouble recharging it, so some of the last shots were done with Izzi’s non-white-balanced phone.) To match the respective colours of light, we dressed Ivan in earthy browns and Izzi in blues and greys.

Artemis recce image

We shot in St. John’s Church in Duxford, Cambridgeshire, which hasn’t been used as a place of worship since the mid-1800s. Unique markings, paintings and graffiti from the middle ages up to the present give it simultaneously a history and a timelessness, making it a perfect match to the clash of eras represented by my two characters. It resonated with the feelings I’d had when I started learning about art and realised the continuity of techniques and aims from me in my cinematography back through time via all the great artists of the past to the earliest cave paintings.

I knew from the tests that long exposures would be needed. Extrapolating from the exposure table, one foot-candle would require a 1/8th of a second shutter with my f1.4 lens wide open and the Fujifilm’s ISO of 400. The 80B has a filter factor of three, meaning you need three times more light, or, to put it another way, it cuts 1 and 2/3rds of a stop. Accounting for this, and the fact that the candle would often be more than a foot away, or that I’d want to see further into the shadows, the exposures were all at least a second long.

As time had become very much the theme of the project, I decided to make the most of these long exposures by playing with motion blur. Not only does this allow a static image – paradoxically – to show a passage of time, but it recalls 19th century photography, when faces would often blur during the long exposures required by early emulsions. Thus the history of photography itself now played a part in this time-fluid project.

I decided to shoot everything in portrait, to make it as different as possible from my cinematography work. Heavily inspired by all the classical art I’d been discovering, I used eye-level framing, often flat-on and framed architecturally with generous headroom, and a normal lens (an Asahi SMC Pentax-M 50mm/f1.4) to provide a natural field of view.

I ended up using my light meter quite a lot, though not necessarily exposing as it indicated. It was all educated guesswork, based on what the meter said and the tests I’d conducted.

I was tempted more than once to tell a definite story with the images, and had to remind myself that I was not making a movie. In the end I opted for a very vague story which can be interpreted many ways. Which of the two characters is the ghost? Or is it both of them? Are we all just ghosts, as transient as motion blur? Do we unwittingly leave an intangible imprint on the universe, like the trails of light my characters produce, or must we consciously carve our mark upon the world, as Ivan does on the wall?

Models: Izzi Godley & Ivan Moy. Stylist: Kate Madison. Assistant: Ash Maharaj. Location courtesy of the Churches Conservation Trust. Film processing and scanning by Aperture, London.

Creating “Stasis”

The Inverse Square Law

If you’ve ever read or been taught about lighting, you’ve probably heard of the Inverse Square Law. It states that light fades in proportion to the square of the distance from the source. But lately I started to wonder if this really applies in all situations. Join me as I attempt to get to the bottom of this…

 

Knowing the law

The seed of this post was sown almost a year ago, when I read Herbert McKay’s 1947 book The Tricks of Light and Colour, which described the Inverse Square Law in terms of light spreading out. (Check out my post about The Tricks of Light and Colour here.)

But before we go into that, let’s get the Law straight in our minds. What, precisely, does it say? Another excellent book, Gerald Millerson’s Lighting for Television and Film, defines it thusly:

With increased distance, the light emitted from a given point source will fall rapidly, as it spreads over a progressively larger area. This fall-off in light level is inversely proportional to the distance square, i.e. 1/d². Thus, doubling the lamp distance would reduce the light to ¼.

The operative word, for our purposes, is “spreads”.

If you’d asked me a couple of years ago what causes the Inverse Square Law, I probably would have mumbled something about light naturally losing energy as it travels. But that is hogwash of the highest order. Assuming the light doesn’t strike any objects to absorb it, there is nothing to reduce its energy. (Air does scatter – and presumably absorb – a very small amount of light, hence atmospheric haze, but this amount will never be significant on the scale a cinematographer deals with.)

In fact, as the Millerson quote above makes clear, the Inverse Square Law is a result of how light spreads out from its source. It’s purely geometry. In this diagram you can see how fewer and fewer rays strike the ‘A’ square as it gets further and further away from the source ‘S’:

Illustration by Borb, CC BY-SA 3.0

Each light ray (dodgy term, I know, but sufficient for our purposes) retains the same level of energy, and there are the same number of them overall, it’s just that there are fewer of them passing through any given area.

So far, so good.

 

Taking the Law into my own hands

During season two of my YouTube series Lighting I Like, I discussed Dedo’s Panibeam 70 HMI. This fixture produces collimated light, light of which all the waves are travelling in parallel. It occurred to me that this must prevent them spreading out, and therefore render the Inverse Square Law void.

This in turn got me thinking about more common fixtures – par cans, for example.

 

Par lamps are so named for the Parabolic Aluminised Reflectors they contain. These collect the light radiated from the rear and sides of the filament and reflect it as parallel rays. So to my mind, although light radiated from the very front of the filament must still spread and obey the Inverse Square Law, that which bounces off the reflector should theoretically never diminish. You can imagine that the ‘A’ square in our first diagram would have the same number of light rays passing through it every time if they are travelling in parallel.

Similarly, fresnel lenses are designed to divert the spreading light waves into a parallel pattern:

Even simple open-face fixtures have a reflector which can be moved back and forth using the flood/spot control, affecting both the spread and the intensity of the light. Hopefully by now you can see why these two things are related. More spread = more divergence of light rays = more fall-off. Less spread = less divergence of light rays = more throw.

So, I wondered, am I right? Do these focused sources disobey the Inverse Square Law?

 

Breaking the law

To find the answer, I waded through a number of fora.

Firstly, and crucially, everyone agrees that the Law describes light radiated from a point source, so any source which isn’t infinitely small will technically not be governed by the Law. In practice, says the general consensus, the results predicted by the Law hold true for most sources, unless they are quite large or very close to the subject.

If you are using a softbox, a Kinoflo or a trace frame at short range though, the Inverse Square Law will not apply.

The above photometric data for a Filmgear LED Flo-box indeed shows a slower fall-off than the Law predicts. (Based on the 1m intensity, the Law predicts the 2m and 3m intensities as 970÷2²=243 lux and 970÷3²=108 lux respectively.)

A Flickr forum contributor called Severin Sadjina puts it like this:

In general, the light will fall off as 1/d² if the size of the light source is negligible compared to the distance d to the light source. If, on the other hand, the light source is significantly larger than the distance d to the light source, the light will fall off as 1/d – in other words: slower than the Inverse Square Law predicts.

Another contributor, Ftir, claims that a large source will start to follow the Law above distances equal to about five times the largest side of the source, so a 4ft Kinoflo would obey the Law very closely after about 20ft. This claim is confirmed by Wikipedia, citing A. Ryer’s The Light Measurement Handbook.

But what about those pesky parallel light beams from the pars and fresnels?

Every forum had a lot of disagreement on this. Most people agree that parallel light rays don’t really exist in real life. They will always diverge or converge, slightly, and therefore the Law applies. However, many claim that it doesn’t apply in quite the same way.

Diagram from a tutorial PDF on light-measurement.com showing a virtual point source behind the bulb of a torch.

A fresnel, according to John E. Clark on Cinematography.com, can still be treated as a point source, but that point source is actually located somewhere behind the lamp-head! It’s a virtual point source. (Light radiating from a distant point source has approximately parallel rays with consequently negligible fall-off, e.g. sunlight.) So if this virtual source is 10m behind the fixture, then moving the lamp from 1m from the subject to 2m is not doubling the distance (and therefore not quartering the intensity). In fact it is multiplying the distance by 1.09 (12÷11=1.09), so the light would only drop to 84% of its former intensity (1÷1.09²=0.84).

I tried to confirm this using the Arri Photometrics App, but the data it gives for Arri’s fresnel fixtures conforms perfectly with an ordinary point source under the Law, leaving me somewhat confused. However, I did find some data for LED fresnels that broke the Law, for example the Lumi Studio 300:

As you can see, at full flood (bottom graphic) the Law is obeyed as expected; the 8m intensity of 2,500 lux is a quarter of the 4m intensity of 10,000 lux. But when spotted (top graphic) it falls off more rapidly. Again, very confusing, as I was expecting it to fall off less rapidly if the rays are diverging but close to parallel.

A more rapid fall-off suggests a virtual point source somewhere in front of the lamp-head. This was mentioned in several places on the fora as well. The light is converging, so the intensity increases as you move further from the fixture, reaching a maximum at the focal point, then diverging again from that point as per the Inverse Square Law. In fact, reverse-engineering the above data using the Law tells me – if my maths is correct – that the focal point is 1.93m in front of the fixture. Or, to put it another way, spotting this fixture is equivalent to moving it almost 2m closer to the subject. However, this doesn’t seem to tally with the beam spread data in the above graphics. More confusion!

I decided to look up ETC’s Source Four photometrics, since these units contain an ellipsoidal reflector which should focus the light (and therefore create a virtual point source) in front of themselves. However, the data shows no deviation from the Law and no evidence of a virtual point source displaced from the actual source.

 

I fought the law and the law won

I fear this investigation has left me more confused than when I started! Clearly there are factors at work here beyond what I’ve considered.

However, I’ve learnt that the Inverse Square Law is a useful means of estimating light fall-off for most lighting fixtures – even those that really seem like they should act differently! If you double the distance from lamp to subject, you’re usually going to quarter the intensity, or near as damn it. And that rule of thumb is all we cinematographers need 99% of the time. If in doubt, refer to photometrics data like that linked above.

And if anyone out there can shed any light (haha) on the confusion, I’d be very happy to hear from you!

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The Inverse Square Law

Choosing an ND Filter: f-stops, T-stops and Optical Density

Imagine this scenario. I’m lensing a daylight exterior and my light meter gives me a reading of f/11, but I want to shoot with an aperture of T4, because that’s the depth of field I like. I know that I need to use a .9 ND (neutral density) filter. But how did I work that out? How on earth does anyone arrive at the number 0.9 from the numbers 11 and 4?

Let me explain from the beginning. First of all, let’s remind ourselves what f-stops are. You have probably seen those familiar numbers printed on the sides of lenses many times…

1      1.4      2      2.8      4      5.6      8      11      16      22

They 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. If you close up the iris to just under 9mm in diameter, you’ll be at f/5.6 (50 divided by 5.6 is 8.93).

A stills lens with its aperture ring marked in f-stops
A stills lens with its aperture ring (top) marked in f-stops

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 will let in twice as much light as (or ‘one stop more than’) one set to f/1.4, which in turn will let in twice as much as one set to f/2, and so on. Conversely, a lens set to f/2 will let in half as much light as (or ‘one stop less than’) one set to f/1.4, and so on.

 

If you think back to high school maths and the Pi r 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.

A Zeiss Compact Prime lens with its aperture ring marked in T-stops
A Zeiss Compact Prime lens with its aperture ring marked in T-stops

Now, earlier I mentioned T4. How did I get from f-stops to T-stops? Well, T-stops are f-stops adjusted to compensate for the light transmission efficiency. Two different f/2 lenses 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 higher than the f-number. For example, Cooke’s 15-40mm zoom is rated at a maximum aperture of T2 or f/1.84.

So, let’s go back to my original scenario and see where we are. My light meter reads f/11. However,  I expressed my target stop as a T-number though, T4, because I’m using cinema lenses and they’re marked up in T-stops rather than f-stops. (I can still use the f-number my meter gives me though; in fact if my lens were marked in f-stops then my exposure would be slightly off because the meter does not know the transmission efficiency of my lens.)

By looking at the series of f-numbers permanently displayed on my light meter (the same series listed near the top of this post, or on any lens barrel) I can see that f/11 (or T11) is 3 stops above f/4 (or T4) – because 11 is three numbers to the right of 4 in the series. I can often be seen on set counting the stops like this on my light meter or on my fingers. It is of course possible to work it out mathematically, but screw that!

CameraZOOM-20140309092150072_zps94e90ea4
A set of Tiffen 4×4″ ND filters

So I need an ND filter that cuts 3 stops of light. But we’re not out of the mathematical woods yet.

The most popular ND filters amongst professional cinematographers are those made by Tiffen, and a typical set might be labelled as follows:

.3      .6      .9      1.2

Argh! What do those numbers mean? That’s the optical density, a property defined as the natural logarithm of the ratio of the quantity of light entering the filter to the quantity of light exiting it on the other side. A .3 ND reduces the light by half because 10 raised to the power of -0.3 is 0.5, or near as damn it. And reducing light by half, as we established earlier, means dropping one stop.

If that fries your brain, don’t worry; it does mine too. All you really need to do is multiply the number of stops you want to drop by 0.3 to find the filter you need. So to drop three stops you pick the .9 ND.

And that’s why you need a .9 ND to shoot at T4 when your light meter says f/11. Clear as mud, right? Once you get your head around it, and memorise the f-stops, this all becomes a lot easier than it seems at first glance.

Here are a couple more examples:

  • Light meter reads f/8 and you want to shoot at T5.6. That’s a one stop difference. (5.6 and 8 are right next to each other in the stop series, as you’ll see if you scroll back to the top.) 1 x 0.3 = 0.3 so you should use the .3 ND.
  • Light meter reads f/22 and you want to shoot at T2.8. That’s a six stop difference (scroll back up and count them), and 6 x 0.3 = 1.8, so you need a 1.8 ND filter. If you don’t have one, you need to stack two NDs in your matte box that add up to 1.8, e.g. a 1.2 and a .6.

 

Choosing an ND Filter: f-stops, T-stops and Optical Density