Shutter speed

Shutter speed is the inverse of exposure time: i.e. it measures how long the camera collects light. This is the parameter with most flexibility: most cameras support shutter speeds at least as fast as 1/200 seconds and at least as slow as 120 seconds.

A short exposure time is desirable when you want to "freeze" motion: for example, when shooting sports. This applies both to motion of the subject and motion of the camera: a common rule of thumb is that when hand-holding you should divide one second by the focal length in mm to get the slowest acceptable shutter speed (e.g. with a 50mm lens you would want a shutter speed of 1/50 s or faster).

A long exposure time is desirable when you want to blur out motion: for example, it's popular for photos of moving water. (Tastes may vary).

The "exposure number" is linear in the exposure time: leave the shutter open for twice as long, and you get twice as much exposure.

ISO

With film, ISO basically comes down to the size of the grain. A film with a small ISO number will look less grainy. Given the same exposure time and aperture, a film with a higher ISO number will look brighter, and the "exposure number" is linear in the ISO.

Digital cameras don't really do grain (although some post-processing might try to add in the appearance of grain); instead they amplify the signal to get the same linear dependency on the ISO number. This is similar to brightening the image in post-production software, which provides some justification for the argument that with digital photography you should prefer to under-expose rather than over-expose.

Note that many digital cameras will allow you to set an ISO number which is faster than the true native ISO of the sensor. That basically means that they're pushing the image after receiving it from the sensor.

Aperture

The aperture is essentially the narrowest part of the path down which the light travels on the way to the sensor. It's a lot easier to understand what it means in a simple lens than a modern lens with multiple elements.

Aperture is always given as a ratio of the focal length: that's why you see e.g. f/5.6. The solidus there indicates division, so a smaller number (e.g. f/1.4) gives a larger aperture than a larger number (e.g. f/16). The ratio given is the diameter of the aperture: the amount of light passing through depends on the area, so you have to square the ratio to find out how it affects the exposure. That's why you see numbers like 1.4 (which is really an approximation of the square root of 2).

A wide aperture (small number) will have a narrow depth of field, so less of a complex scene will be in focus. A narrow aperture (large number) will have a deeper field, with more of the scene in focus. However, a physical effect called diffraction means that a very narrow aperture will blur everything slightly. For maximum sharpness across the scene you typically want an aperture in the region of f/8 or f/11.

Manual versus automatic

Automatic modes work by using a light meter to estimate the correct exposure number for the scene and then adjusting one or more of the three parameters to try to achieve it. This works well when the metering works well and the constraints that you've placed on the parameters are suitable. It is still necessary to sanity check: if you're in A mode and have disabled automatic ISO adjustment, you may find that the exposure time is 1s and you need to adjust the aperture or ISO.

Manual mode works well when you know that the light meter will get the wrong value. For example, in bird photography the subject is often small and fast-moving. Automatic exposure will probably expose for the background, not the subject. Some bird photographers therefore swear by metering against the sky and then adjusting by a given amount (which may depend on weather conditions).

posted about 4 years ago

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Peter Taylor‭

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