How Cinematographers Set Their Exposure

WHAT IS EXPOSURE?

You’ve no doubt seen how some movies look darker than others. Comedies may be bright and colourful, while dramatic crime films may be dark and moody. This is, to a large part, determined by how cinematographers set their exposure: the level of brightness across a frame.

Although it may seem simple on the surface, being able to expose properly is one of the most crucial skills that every cinematographer needs.

Exposure is determined by six factors: the sensor’s sensitivity or ISO, the shutter speed, the lens’ aperture, the frame rate, ND filters and of course the overall amount of light in the scene that is being photographed. Before we talk about how to get to the correct exposure it’s important to be aware of a few terms. 

The amount of light present in an image will almost never be completely even across the whole frame. Part of the job of the cinematographer is to use lighting to create different pockets of light and shadow in an image - called contrast. Sometimes that contrast might be light, other times it may be strong. 

The difference between the brightest and darkest part of the image is called the contrast ratio. Traditionally in cinematography this refers to the difference in illumination between the brightness of the key light and brightness of the fill.

Similarly, a camera also has a threshold of how much of the difference between light and dark it can capture before the information turns to pure white or pure black. This is called dynamic range. 

Modern digital cinema cameras usually have around 15 to 17 stops of dynamic range, while consumer video cameras may have less than 14 stops. This is one of the reasons that it may be more difficult to achieve a balanced exposure on cheaper cameras. 

If the strength of the contrast ratio exceeds the dynamic range of the camera, then the image will either blow out - and turn to pure white - or it will get crushed - and turn to pure black. 

Usually cinematographers like to light scenes so that the total exposure falls within the dynamic range of the camera. In other words, they like to create images in which the highlights do not blow out, and in which the shadows are not completely crushed. In a similar way to how the human eye perceives brightness in the real world.

HOW TO FIND THE RIGHT EXPOSURE?

So, we know what exposure is, but how do we find the right exposure using a camera?

Cinematographers, or sometimes the DIT, regularly use different tools to determine the best exposure - which we’ll get to a bit later. But good exposure on digital cameras can also be achieved just by referring to the image on the monitor.

The thing is, what is considered a ‘good’ exposure is a bit subjective. For example, The Batman is lit and exposed at a very low level of illumination, while a comedy like Palm Springs exposes at a much brighter level. I’d say this lower level of exposure helps to accentuate the moody, emotionally dark tone in Batman, while the brighter exposure is, literally, lighter, and brighter in tone.

It doesn’t necessarily mean an image is bad, or incorrectly exposed if a small part of the frame is blown out, or crushed to black. As long as its exposure suits the tone of the story and the environment being photographed.

Although exposure is subjective, most cinematographers still try to avoid one thing - blowing out highlights to pure white. There are always exceptions to this rule and situations where this might be impossible to avoid, but for the most part a good rule of thumb with exposure is to try to set it so that any large, bright part of the frame maintains detail.

This is called exposing for, or protecting, the highlights.

Cinematographers using this technique will first dial down the exposure to the point where the brightest highlights still preserve colour information. This may mean that other areas of the image are now too dark. 

To counteract this, DPs may then add light - using artificial sources - to lift up the brightness of the shadows until there is detail present. 

They may also use lighting to bring down the levels of any highlights to an acceptable level, for example using a dimmer on a practical lamp, or adding an ND gel or a net to a window. 

That’s why the best way to control the contrast ratio in an image is usually by lighting.

Sometimes cinematographers may be forced to blow out bright highlights that they can’t control which take up a small portion of the image, such as windows, flames or practical fixtures. 

As a side note, film stocks tend to deal with overexposure of highlights better than digital cameras do. Film gives more of a soft halation, a gradual fall off and renders highlights in a creamier colour, as opposed to the uglier, pure white of digital. 

However, there are ways to counteract blown digital highlights, both in the grade, as well as by trying to introduce a softer highlight rolloff using diffusion filters.

EXPOSURE TOOLS

Although exposing by just looking at the monitor is popular, it also comes with a few downsides. The first of which is that, unless you’re working with a specially calibrated monitor, every screen produces a different level of brightness and represents colours and exposure values in different ways. This could trick you into exposing incorrectly. 

Light conditions on set may also affect how you see exposure. Even if you’re working with an older monitor like a TV Logic 056, which has lovely, accurate colours, your exposure may still be off as the monitor has a very dim picture brightness in outdoor situations.

To avoid making exposure mistakes, cinematographers either use physical tools, like a light meter, or digital software tools built into the camera such as a waveform or false colour - which is the most common exposure tool on high end cinema cameras from Arri and Red.

When activated, false colour displays images on a monitor in a specific colour spectrum - ranging from a warm red, to a cool purple or dark blue - with green sitting somewhere in the middle. Each colour represents an IRE value - a measure of brightness on a scale of 0 - being complete black - to 100 being complete white.

Cinematographers can use false colour to check that their highlights aren’t red and blowing out and that their shadows aren’t purple and getting crushed to black. People often recommend that to get perfect skin tones you should expose to where the skin tones turns green or a light grey on the false colour reading.

However, as I mentioned before, some cinematographers may want to disregard this and expose darker in the dark grey to blue range to achieve a moodier feel. 

Another useful exposure tool to understand is the waveform. This is made up of a vertical axis that expresses exposure in IRE values from 0 to 100 and a horizontal axis that represents the width of the frame.

For example if there is one bright highlight in the centre of the image it will be represented by a spike in IRE value in the centre of the waveform.

Like with false colour, a waveform can also be consulted to see whether areas of the image are exceeding 100 IRE and blowing out, or sitting on 0 IRE and getting crushed.

Usually when consulting a monitor for exposure it’s good to use a LUT to expose with rather than looking at the log image. Because a log image is so flat it may be easy to under or overexpose it, while still seeing detail in the log. But later in post production you may find that when you add a LUT or grade it that it is under or over exposed and doesn’t have enough detail to work with.

CONTROLLING EXPOSURE

As I mentioned before, there are six exposure variables. Each of these variables can be manipulated to arrive at an acceptable exposure, however, some of these variables also have other effects on the image. 

For example frame rate changes the speed at which motion is captured, the shutter angle changes how blurry that motion is, the ISO affects how grainy or noisy the image is, and the aperture affects to what extent the background is out of focus.

For this reason, many cinematographers maintain the same or a similar frame rate, shutter, aperture and ISO throughout a project and change the other two variables - the amount of light on a set and the strength of the ND filter - to control exposure.

ND or neutral density filters are made up of glass which reduces the amount of light that hits the sensor without changing the colour characteristics of that light. 

When lighting a scene it’s important to ensure that there is enough light to properly expose the image. Rather have too much light than too little light. You can always cut down the amount of light by using ND filters, but if a space is too dark you can’t get a brighter exposure without increasing the amount of grain or noise by lifting the camera’s sensitivity.

The best way to go about controlling your exposure with a cinema camera is to first ensure your ISO, shutter and frame rate are correctly dialled in. Then stop down the aperture on the lens until you arrive at an exposure which you like. This may leave the lens at a stop of, for example T/ 8. 

Next, you need to determine what stop or aperture you would like to shoot at. For example maybe you’d like to shoot at T/ 2 to get a shallow depth of field.

You then need to calculate the difference in stops between T/8 and T/2 - one, two, three, four. This means you need to add four stops of ND to get to a shooting aperture of T/2.

Each stop of ND filter is measured in increments of 0.3. So four stops of ND would be ND 1.2. This means you can add an ND 1.2 either with internal ND filters or with external glass ND filters in a mattebox and set the lens to T/2. 

You now have the correct exposure, while also having your desired aperture of T/2 that produces a beautiful, shallow depth of field.