Determining and Setting Exposure

Exposure is a fancy word for how bright your final image appears. The more ‘exposed’ a sensor is to light the brighter the picture. Too bright and your digital sensor will be overexposed, resulting in clipping (sensor is saturated with light and everything just registers as white). Too dark and the underexposed sensor will pollute your image with noise. You’ll hear ETTR (expose to the right) mentioned frequently. This is the idea of exposing a sensor as bright as possible without clipping and then bringing the brightness back down in post. Doing so reduces the amount of noise in the image.

Aperture

This is simply how open the lens. Like the iris of an eye or the window shade of an airplane, if you open the aperture you let more light in. The cost of a lens will often be related to its aperture. Lenses with higher f-numbers allow a lot of light through, work better in low light, and generally cost more. Large apertures create more shallow depth of field and small apertures can soften the image due to diffraction.

Shutter Speed

Shutter speed is simply how long your digital sensor is exposed to the light coming through the lens’ aperture. A fast shutter speed will result in a darker image since not much light has had time to hit the sensor. Because of this, you’ll get less motion blur since your subject couldn’t have moved much in the time the shutter was open. A slow shutter speed results in a brighter image and increased motion blur. Sometimes extremely slow shutters can be used for dramatic effect.

ISO

This acronym (International Standards Organization) can most easily be thought of as how sensitive the sensor is to light. Most digital sensors will have a sweet spot or “native ISO” where the camera achieves its greatest dynamic range. Some cameras are considered ISO independent, meaning you can shoot at a lower ISO and boost the image in post without too much degradation from noise.

So how do these work together?

The f-stop Scale

1.4

2

2.8

4

5.6

8

11

16

Metering

Many classic cinematographers critique the meter-obsessed newcomers who religiously park their exposure safely in the middle of their scopes. There are often times when a dark dramatic frame will be visually appealing, but not look like a usable exposure on a histogram or waveform. Most of these come from the historical practice of determining final exposure on the camera however. In the digital world, post manipulation of the image allows you to “shoot safe” and create the dramatic exposure in the comfort of a color suite.

The camera generally has a meter built in to guide you with setting exposure. This meter is a reflective meter that measures the light coming through the lens. Incident meters can be used at the location of the subject to measure the amount of light hitting any point in space. The in-camera meter makes its decisions based on what portion of the frame you’ve assigned it to use to evaluate exposure. “Spot metering” will only use a small portion of the frame to determine exposure while a “matrix metering” will consider the majority of the image and will better average the whole. The meter generally tries to put your image right in the center of the exposure range, the middle of the histogram, or 18% gray.

Incident metering can be helpful in determining lighting ratios or the difference between two lights. A common example would be determining how strong your key light is relative to your fill light.

Digital cameras shooting log formats reduce the midtone exposure in order to make room for more highlights and increased dynamic range. When using these recording formats, do not try exposing an 18% gray card at 50%.

Exposure for Video

The principle of aperture applies the same to photo and video. Shutter speed is a bit simpler in the video world. Since we usually shoot at 24 frames per second for a “cinematic look”, and the shutter opens and closes twice-per exposure, all you have to do is double your framerate to get the denominator (bottom number) of the appropriate shutter speed. So a 1/48 shutter works well for 24 frames per second. If you’re shooting higher frame rates then the same rule applies–a 96 frames-per-second shot benefits from a shutter-speed of at least 1/192.

When the shutter speed is increased beyond this value the individual frames will appear too sharp and the motion will look jittery. Just make sure it’s a conscious artistic decision if you do this.