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Exposure explained

For your film to look good, you need to make sure that it’s not too bright (overexposed) or too dark (underexposed). The easy, lazy way to do this is to let the camera work it out. But to shoot really good video you need to set it yourself.

You can adjust the exposure by changing

  • how sensitive the camera is to light (ISO)
  • the amount of light the lens lets through (aperture)
  • the amount of time the light gets through (shutter speed)

If you make one of these bigger, you can make one of the others smaller. So you could let twice as much light in for half as much time.

Sensitivity (ISO)

The camera’s sensitivity to light is measured in ISO. Low ISO is around 100 or less, medium is around 200-400, high is over 400. If the ISO gets too high the image quality will get worse: it’ll be less sharp, with worse colours, and speckles of multicoloured ‘noise’.

For good quality at high ISO, you need a camera with a big light-capturing sensor. So most DSLRs and mirrorless cameras will shoot acceptable video at high ISO settings (800, 1600 or even higher), but with compact cameras the quality quickly gets worse as you crank up the ISO. You’ll get the best quality filming at the camera’s base ISO (you can usually find this out with an online search).

Aperture

You can also change how much light comes through the lens. Most cameras (but not smartphones) have an iris which lets you adjust the size of the hole in the lens. The size of this hole is called the aperture, and it’s measured in f-stops. An aperture of f/2 lets in twice as much light as f/2.8.

A wider aperture such as f/2 has less ‘depth of field’ than a smaller aperture such as f/5.6 (with the same lens). In other words, less of the scene will be in focus in front of and behind the subject. Arty ‘shallow focus’ shots with blurred backgrounds can look great, but you have to be really careful to keep the main subject sharp. If the subject is moving around a lot it’s better to use a smaller aperture like f/5.6 to keep more of the scene in focus.

But don’t close the aperture down too far, particularly with a small camera, as you’ll lose sharpness because of an effect called diffraction. Usually with a DSLR you want to avoid apertures smaller than f/8 or f/11 depending on the lens.

  • Why does a bigger number mean a smaller hole?
    Because f-stops are fractions: it’s the diameter (width) of the iris, divided by the focal length of the lens. So if you’re using a 50mm lens, at f/2 the iris is 50mm divided by two: 25mm.
  • Why do aperture numbers follow an odd sequence (1.4, 2, 2.8, 4, 5.6 etc)?
    An f-stop is based on the diameter of the iris, but to halve the amount of light it’s the area of the hole that has to halve.

T-stops

Some cine lenses are marked in T-stops rather than F-stops. T-stops measure the amount of light that gets through the lens, not the size of the aperture. If you switch between lenses at the same F/stop, the exposure might change because different amounts of light could be absorbed or reflected by the lens glass. But with T-stops, you can switch between lenses with no change in exposure.

Shutter speed

The third thing you can change is how long the sensor is exposed to the light. Shutter speeds are measured in fractions of a second. High shutter speeds (eg 1/1000 of a second) will freeze motion in the individual frames of your video, but the movement may not look very smooth (see below).

Using aperture, shutter speed and sensitivity together

You can balance these three settings against each other. Say you’ve set the aperture at f/2 with a shutter speed of 1/250, but you decide you need more depth of field. You could stop down the aperture to f/2.8 but use a slower shutter speed of 1/125. You’d be letting in half as much light for twice as long, so the exposure would be the same.

What’s different about video…

This will all be familiar to experienced still photographers; but with video, there’s a catch. To make motion look smooth, you need the shutter speed to be half the frame rate. (You don’t need to worry about this if your scene doesn’t include much movement.) If your video is 25 frames a second, the shutter speed should be 1/50th.This means that you have to leave shutter speed alone, and change just aperture and ISO sensitivity.

If you need to keep your aperture wide – e.g. in bright sunlight, or to get shallow focus effects – you can cut down the amount of light by using a special neutral density (ND)  filter. Most pro video cameras have built-in ND filters, but for system still cameras you need to buy them separately. You can get them individually or in sets. Some SLR filmmakers use variable ND filters. These are expensive – and can give odd effects with wide-angle lenses – but they let you smoothly change the exposure without constantly swapping filters.

…and phones

Smartphones like the iPhone have fixed apertures. Because their sensors are so tiny, they still have quit a lot of depth of field, even at f/1.7 or f/2. But having a fixed aperture means that you can only use shutter speed and ISO to vary the exposure. Most smartphones automatically choose the best combination, though you can select and vary them manually with the Filmic Pro camera app. And if you have a case with a thread for add-on lenses like the iOgrapher, you can fit a variable ND filter.

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