6 Ways to Judge Exposure

Exposing the image correctly is one of the most important parts of a cinematographer’s job. Choosing the T-stop can be a complex technical and creative decision, but fortunately there are many ways we can measure light to inform that decision.

First, let’s remind ourselves of the journey light makes: photons are emitted from a source, they strike a surface which absorbs some and reflects others – creating the impressions of colour and shade; then if the reflected light reaches an eye or camera lens it forms an image. We’ll look at the various ways of measuring light in the order the measurements occur along this light path, which is also roughly the order in which these measurements are typically used by a director of photography.

 

1. Photometrics data

You can use data supplied by the lamp manufacturer to calculate the exposure it will provide, which is very useful in preproduction when deciding what size of lamps you need to hire. There are apps for this, such as the Arri Photometrics App, which allows you to choose one of their fixtures, specify its spot/flood setting and distance from the subject, and then tells you the resulting light level in lux or foot-candles. An exposure table or exposure calculation app will translate that number into a T-stop at any given ISO and shutter interval.

 

2. Incident meter

Some believe that light meters are unnecessary in today’s digital landscape, but I disagree. Most of the methods listed below require the camera, but the camera may not always be handy – on a location recce, for example. Or during production, it would be inconvenient to interrupt the ACs while they’re rigging the camera onto a crane or Steadicam. This is when having a light meter on your belt becomes very useful.

An incident meter is designed to measure the amount of light reaching the subject. It is recognisable by its white dome, which diffuses and averages the light striking its sensor. Typically it is used to measure the key, fill and backlight levels falling on the talent. Once you have input your ISO and shutter interval, you hold the incident meter next to the actor’s face (or ask them to step aside!) and point it at each source in turn, shading the dome from the other sources with your free hand. You can then decide if you’re happy with the contrast ratios between the sources, and set your lens to the T-stop indicated by the key-light reading, to ensure correct exposure of the subject’s face.

 

3. Spot meter (a.k.a. reflectance meter)

Now we move along the light path and consider light after it has been reflected off the subject. This is what a spot meter measures. It has a viewfinder with which you target the area you want to read, and it is capable of metering things that would be impractical or impossible to measure with an incident meter. If you had a bright hillside in the background of your shot, you would need to drive over to that hill and climb it to measure the incident light; with a spot meter you would simply stand at the camera position and point it in the right direction. A spot meter can also be used to measure light sources themselves: the sky, a practical lamp, a flame and so on.

But there are disadvantages too. If you spot meter a Caucasian face, you will get a stop that results in underexposure, because a Caucasian face reflects quite a lot of light. Conversely, if you spot meter an African face, you will get a stop that results in overexposure, because an African face reflects relatively little light. For this reason a spot meter is most commonly used to check whether areas of the frame other than the subject – a patch of sunlight in the background, for example – will blow out.

Your smartphone can be turned into a spot meter with a suitable app, such as Cine Meter II, though you will need to configure it using a traditional meter and a grey card. With the addition of a Luxiball attachment for your phone’s camera, it can also become an incident meter.

The remaining three methods of judging exposure which I will cover all use the camera’s sensor itself to measure the light. Therefore they take into account any filters you’re using as well transmission loss within the lens (which can be an issue when shooting on stills glass, where the marked f-stops don’t factor in transmission loss).

 

4. Monitors and viewfinders

The letter. Photo: Amy Nicholson

In the world of digital image capture, it can be argued that the simplest and best way to judge exposure is to just observe the picture on the monitor. The problem is, not all screens are equal. Cheap monitors can misrepresent the image in all kinds of ways, and even a high-end OLED can deceive you, displaying shadows blacker than any cinema or home entertainment system will ever match. There are only really two scenarios in which you can reliably judge exposure from the image itself: if you’ve owned a camera for a while and you’ve become very familiar with how the images in the viewfinder relate to the finished product; or if the monitor has been properly calibrated by a DIT (Digital Imaging Technician) and the screen is shielded from light.

Most cameras and monitors have built-in tools which graphically represent the luminance of the image in a much more accurate way, and we’ll look at those next. Beware that if you’re monitoring a log or RAW image in Rec.709, these tools will usually take their data from the Rec.709 image.

 

5. Waveforms and histograms

These are graphs which show the prevalence of different tones within the frame. Histograms are the simplest and most common. In a histogram, the horizontal axis represents luminance and the vertical axis shows the number of pixels which have that luminance. It makes it easy to see at a glance whether you’re capturing the greatest possible amount of detail, making best use of the dynamic range. A “properly” exposed image, with a full range of tones, should show an even distribution across the width of the graph, with nothing hitting the two sides, which would indicate clipped shadows and highlights. A night exterior would have a histogram crowded towards the left (darker) side, whereas a bright, low contrast scene would be crowded on the right.

A waveform plots luminance on the vertical axis, with the horizontal axis matching the horizontal position of those luminance values within the frame. The density of the plotting reveals the prevalence of the values. A waveform that was dense in the bottom left, for example, would indicate a lot of dark tones on the lefthand side of frame. Since the vertical (luminance) axis represents IRE (Institute of Radio Engineers) values, waveforms are ideal when you need to expose to a given IRE, for example when calibrating a system by shooting a grey card. Another common example would be a visual effects supervisor requesting that a green screen be lit to 50 IRE.

 

6. Zebras and false colours

Almost all cameras have zebras, a setting which superimposes diagonal stripes on parts of the image which are over a certain IRE, or within a certain range of IREs. By digging into the menus you can find and adjust what those IRE levels are. Typically zebras are used to flag up highlights which are clipping (theoretically 100 IRE), or close to clipping.

Exposing an image correctly is not just about controlling highlight clipping however, it’s about balancing the whole range of tones – which brings us to false colours. A false colour overlay looks a little like a weather forecaster’s temperature map, with a code of colours assigned to various luminance values. Clipped highlights are typically red, while bright areas still retaining detail (known as the “knee” or “shoulder”) are yellow. Middle grey is often represented by green, while pink indicates the ideal level for caucasian skin tones (usually around 55 IRE). At the bottom end of the scale, blue represents the “toe” – the darkest area that still has detail – while purple is underexposed. The advantage of zebras and false colours over waveforms and histograms is that the former two show you exactly where the problem areas are in the frame.

I hope this article has given you a useful overview of the tools available for judging exposure. Some DPs have a single tool they rely on at all times, but many will use all of these methods at one time or another to produce an image that balances maximising detail with creative intent. I’ll leave you with a quote from the late, great Douglas Slocombe, BSC who ultimately used none of the above six methods!

I used to use a light meter – I used one for years. Through the years I found that, as schedules got tighter and tighter, I had less and less time to light a set. I found myself not checking the meter until I had finished the set and decided on the proper stop. It would usually say exactly what I thought it should. If it didn’t, I wouldn’t believe it, or I would hold it in such a way as to make it say my stop. After a time I decicded this was ridiculous and stopped using it entirely. The “Raiders” pictures were all shot without a metre. I just got used to using my eyes.

6 Ways to Judge Exposure

“Barry Lyndon”: The Full Story of the Famous f/0.7 Lenses

After seeing Barry Lyndon (1975) on the big screen this week, I felt compelled to write a blog post about its cinematography. But what aspect of the cinematography? The painterly look? The many zooms? The use of natural light?

What I knew for certain is that I should definitely not write about the entirely candlelit scenes lensed on f/0.7 Nasa glass, because everyone knows that story. However, reading the vintage American Cinematographer article and some other material, I found the details surrounding this groundbreaking use of high-speed lenses so interesting that I decided to do it anyway.

 

The Vision

Barry Lyndon is the 18th century tale of a low-born Irishman who strives – through various misadventures, and ups and downs of fortune – to become a gentleman. The key visual influence of director Stanley Kubrick and DP John Alcott, BSC were the great painters of the story’s era, such as Vermeer.

Next week’s post will look at this painterly influence in Barry Lyndon more closely, but for now the important thing is the use of candlelight on those classical canvases, and Kubrick’s desire to replicate that look. According to lens expert Ed DiGuilio, who was tasked with adapting the f/0.7 glass for Lyndon, Kubrick “wanted to preserve the natural patina and feeling of these old castles at night as they actually were”.

Typically in movies, a candle in frame may motivate the lighting, but most of the illumination on the actors actually comes from an orange-gelled lamp just out of frame. Kubrick wasn’t interested in shooting Lyndon that way. He wanted all the light in those night interior scenes to genuinely come from the candles themselves.

 

The Problem

How much light does a candle shed? Conveniently, there is a unit of illumination called the foot-candle. One foot-candle is the amount of light received from a standard candle one foot away. Without going into the detail of what a “standard” candle is, it is enough for our purposes to say that the scene below has a key light of about three foot-candles…

… because there are three candles, about a foot away from the actor’s face. (The level of your key light, and consequently where you set your aperture, is almost always measured at your subject’s face, as that is usually the focus of the shot and the most important thing to get correctly exposed. This is why we DPs are always waving light meters in actors’ faces.)

If we look at an exposure table, such as this one, we can see that a three foot-candle key can be correctly exposed with an aperture of T1.4 and an EI (exposure index) of 800. Today that would be no problem, with many digital cameras having a native EI of 800, and the availability of fast lenses like Zeiss Master Primes and Super Speeds.

In the mid-seventies however, long before the advent of digital cameras, things were not so simple. Kubrick and Alcott had little choice but to shoot on Eastman Kodak 100T 5254. Those first three digits denote the film stock’s exposure index: 100. Alcott pushed the stock (brought the brightness up during processing) one stop, re-rating it to an EI of 200. But it still needed four times more light, or two stops more light than our modern-day Alexa or Red. (Check out my post on f-stops and T-stops if you’re getting lost.)

If we’re losing two stops on the EI, we need to gain two stops on the aperture to compensate. And two stops up from T1.4 is T0.7. You may notice that T0.7 isn’t on that table I linked to. This is because a lens with such a large relative aperture pretty much doesn’t exist.

Pretty much…

 

The Solution

Kubrick obsessively researched the problem. He eventually discovered that Nasa had commissioned Carl Zeiss to build ten Planar 50mm f/0.7 stills lenses in the sixties, which were used to take photos of the dark side of the moon. (I was unable to find out the T-stop of these lenses, but I’ll assume it was close enough to T0.7 for it to make little difference to my calculations above.) The developments leading to these lenses stretched back through Nazi military applications during WW2 all the way to the late Victorian era, when the double-Gauss cell at the core of the lenses was first invented.

Anyway, Kubrick promptly bought three of the Zeiss Planars. He liked to own equipment himself, rather than hire it in, and to this end he had also purchased at least one Mitchell BNC camera. As befits Kubrick’s perfectionism, these were perhaps the world’s most precisely engineered cameras, previously used for special effects work.

This is where Ed DiGuilio comes in: “[Kubrick] called one day to ask me if I thought I could fit a Zeiss lens he had procured… to his BNC.” It wasn’t simply a case of the f/0.7 glass having the wrong mount. The rear element was so large and needed to be so close to the film plane that DiGuilio had to extensively modify the camera, literally cutting parts out of it.

Ed DiGuilio (left), President of Cinema Products Corporation, working on adapting a zoom lens for Kubrick’s Mitchell BNC

Once this was done, extensive testing ensued. The focus scale (distances marked on the barrel) had to be calibrated from scratch, and indeed the focus ring was re-engineered to allow the precision focusing that the lens’ tiny depth of field would require. Whereas the focus ring on a stills lens will turn about 90° to go from infinity to close focus, and the ring on a cine lens might turn 270°, the rings on these unique Planars now turned a whopping 720° – two whole revolutions!

50mm is a very useful lens length for close-ups, but Kubrick understandably wanted a wider option as well. Accordingly, DiGuilio located an adapter designed to adjust the throw of cinema projector lenses. Mounted onto one of the 50s, it gave an effective focal length of 36.5mm with only very minor light loss. A 24mm version was also tested, but Kubrick disliked the amount of distortion in its images, and rejected it.

 

The Execution

The colour brown and the trousers of Doug Milsone, Barry Lyndon‘s focus puller, cannot have been strangers to each other. Imagine trying to hold focus on this dolly-back at f/0.7!

By my calculations (which were difficult, because most depth of field tables/calculators don’t go to f/0.7!) an MCU on Kubrick’s 50mm Planar with the subject at 2.5m (8.2ft) and the iris wide open would have had a depth of field of about 43mm (1.7″). To get this same depth of field at f2.8, a popular working stop for cinematographers today, the subject would have to be just 1m (3.3ft) from the sensor plane, which would be a biggish close-up. And remember that focus monitors, peaking and Cine Tape did not exist in the seventies.

To give Milsone a fighting chance, a unique system of focus assist was developed. While the main camera shot an actor from the front, a CCTV camera captured them in profile. This profile image was piped to a monitor, over which a grid was placed. This grid was marked off with distances so that Milsone could see how much the actor had moved by, far more accurately than judging it by eye from beside the lens.

Another problem thrown up by the low-light cinematography was with the viewfinder. Interestingly, the Mitchell BNC was a non-reflex camera, meaning that it didn’t have a mirror on the shutter, reflecting the image to the viewfinder when the shutter was closed. Instead, the camera body racked over to one side to allow the viewfinder to get an image during line-ups and rehearsals, and when it was actually rolling the operator got their images from a side viewfinder with its own lens – just like in a disposable 35mm stills camera. The original prism-based viewfinder on Kubrick’s Mitchell BNC suffered from far too much light loss for a candlelit image to be visible through it, so it was replaced with a mirror-based viewfinder adapted from a Technicolor camera.

The shots resulting from all of these technical challenges are quite soft to the modern eye, but I think that only adds to their beauty. Barry Lyndon captured the exquisite fragility of candelight, and 42 years on the images are still unique and captivating.

“Barry Lyndon”: The Full Story of the Famous f/0.7 Lenses