Exposure

Exposure: Getting the Light Right

Exposure is the amount of light used to make the photograph. These days almost all cameras will set the exposure for you automatically. Better cameras will allow you to adjust it.

Technical Know How

For correct exposure, the combination of film ISO (which used to be called ASA), aperture and shutter speed must suit the amount of light.

The Film ISO

Traditional film comes in a variety of "speeds", from 50 ISO to 1000 ISO. The commonest films are 100 or 400 ISO. The lower the ISO number, the more light you need to make the picture and the finer the grain on the slide or negative. Most of my exhibition photos were taken on 50 ISO film, because I wanted the grain to be invisible even on a 60cm x 40cm enlargement. That meant that I needed a slow shutter speed (see below) and so I had to use a tripod or other support which takes time to set up. That's OK. The mountains weren't going anywhere. But it makes 50 ISO film unsuitable for sports photography, for example.

Good digital cameras allow you to set the ISO. The number of pixels stays the same regardless, but a low ISO setting means less noise and a better quality picture.

The Shutter Speed

The shutter speed is the amount of time the hole at the front of the camera is open to allow light through to the film. Professional cameras allow you to set speeds from 1/1000th of a second through to a whole second. Fairly obviously, a 1/500th of a second lets through half as much light as a 1/250th.

For historical reasons, photographers talk about light in stops. Every time you double or halve the amount of light, that's a stop. A 1/125th is a stop more than a 1/250th, and two stops less than a 1/30th.

Shutter speed is important when the subject (or part of it) is moving. The faster the movement and the slower the shutter speed, the more it will blur. See more under Know Why.

The Aperture

This is the size of the hole that opens to let the light through to the film. A bigger hole means more light.

The numbers look crazy, (f11, f8, f5.6...) but there's a reason for it.

The "f" stands for "focal", because it depends on the focal length of the lens. F8 on a telephoto lens is a bigger hole than f8 on a wide angle, but they let the same amount of light through to the film.

And those funny numbers are diameters. In order to get twice the area (and so twice the light, one stop more) you increase the diameter by the square root of 2. This means that for every two stops, the number halves: f16, f11, f8, f5.6, f4, f2.8, f2...See?

The aperture also affects how much of your subject is in focus.

Small apertures (high numbers, like f16) mean that you have a lot of depth of field. With a wide angle, you can have everything in focus from 1 metre to infinity.
Large apertures (low numbers, like f2.8) mean you have very little depth of field. You have to choose which bit to have in focus.

For more on this, seeKnow Why below.

A wide aperture gives a small depth of field, throwing the background out of focus. This was taken at f4.

Putting it together

All this means that there are a lot of ways of getting the same exposure. All of the following will give correct exposure on a sunny day.

	ISO	Aperture	Shutter	Comment
	100	f16	1/125th	All the other comments refer to this setting
invisible	400	f16	1/500th	Film two stops more sensitive, two stops less shutter speed.
invisible	100	f8	1/500th	Two stops more aperture, two stops less shutter speed.
invisible	50	f4	1/1000th	Film one stop less sensitive, four stops more aperture, three stops less shutter speed.

You would have a different set of possibilities on a cloudy day, but there would still be a lot of them.

Know Why

So if f16 and a 1/125th gives the same exposure as f5.6 and 1/1000th why bother to have two sets of controls? Why not have just one?

Because although the exposure stays the same, other things change.

Many cameras allow you to set the aperture and the camera will set a suitable shutter speed for you. This is called aperture priority. Alternatively, you can set the shutter speed, and the camera selects a matching aperture. This is shutter priority. Or you can set both yourself, which is manual or fully manual.

A fast shutter speed stops motion. If you want a photograph of a waterfall to show the spray as separate drops in midair, use the fastest shutter speed you can. This means a wide aperture. (If you're in the shade, you'll need fast film or a flash.) Conversely, if you want the water to blur, use a slow shutter speed. This means a small aperture.

Remember that slow shutter speeds might mean that the camera moves during the exposure. This is called camera shakeand it means that nothing in the picture is sharp. If the shutter speed is below about 1/60th, you'll need to support the camera with a tripod or something else.

To blur or not to blur is a creative decision. If you use 1/60th photograph a rally car as it whizzes by, the resulting blur will give a good impression of speed, but you might not even be able to tell which car it was. If you use a 1/1000th you might just catch the expression on the driver's face if you're lucky. It's your choice.

Similarly, it's up to you how much of the picture you want to have in sharp focus. I like landscapes with foreground detail to lead your eye into the picture. That means I need a small aperture, which in turn means a slow shutter speed and a tripod, especially since I like to use slow film. But when I photograph flowers, I like a plain background. Sometimes I use a wall or the sky, but when that's not possible, I put the background (and sometimes the foreground) out of focus by using the widest possible aperture. Of course that means a fast shutter speed to compensate, which is useful if the wind's blowing.

To summarise: If you've got a cheap digital camera, or if you've already loaded the film, you can't control the ISO. But there are always several combinations of aperture and shutter speed which will give the correct exposure. However, they each give different amounts of blur, and different depths of field. This gives you a lot of creative possibilities to play with.

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