What do we mean by exposure?
Here are three definitions of exposure. I think each of them is helpful.
• Exposure is the amount of light falling on the sensor or film, over a period of time.
• Exposure is the total amount of light that reaches the camera's sensor, or film.
• Exposure is 'light intensity x time'.
But what is a 'correct' exposure?
To some degree a correct exposure is a subjective matter, but within certain boundaries. Generally we would consider a photograph as over-exposed when the highlights are 'blown' or 'burnt out' (there is no detail left in the highlights). Likewise a photograph with solid black shadow areas would normally be considered under-exposed. It is worth saying however that there may be cases where you might aim for a very dark or very light result in a photograph.
The exposure triangle
A correct exposure involves a combination of three important factors: Aperture, Shutter Speed and ISO. In his excellent book 'Understanding Exposure' Bryan Paterson describes this as 'The Photographic Triangle'.
Let's talk about ISO for a minute
ISO is a measure of the sensitivity of your camera's sensor (or film). The lower the number – 100 ISO is the lowest setting (or 'base ISO') on most digital cameras – the less sensitive it is to the light falling on it. The higher the number the more sensitive it is to the light. So your camera were set on a low ISO and mine on a high ISO, and we were shooting the same subject at the same time, I would need less light to land on my sensor that you would on yours.
The lower the ISO the higher the quality.
As the sensitivity of a sensor increases so does the visual 'noise' on the image. This is a little like the grain on a film, but less attractive. Noise is easiest to see in areas of shadow, but it can appear all over an image. The higher the ISO the more noise on the image.
It is best to shoot at your camera's base ISO whenever possible.
So why should I change the ISO?
Because there will be times when shooting conditions are such that you cannot get the shot you want unless something changes. Let's say its a very dull overcast day and you have your camera set to 100 ISO. When you look at your light meter's display you see that its telling you that to get a 'balanced' exposure means you will have to shoot at maximum aperture (lets say at f4) and with a shutter speed of 1/15 second. But hand-holding the camera for 1/15 second will almost certainly result in camera shake and a blurred image – probably not what you want.
So what are your options?
You could put the camera on a tripod so that shooting at 1/15 second isn't a problem as far as camera-shake is concerned. This isn't always feasible – whether because your tripod happens to be in the car or at home in a cupboard… or because your subject isn't suitable for shooting in this way – like an active child or someone moving around and so on.
You could introduce additional light into the scene, for example with flash or some other light source. Again, not always feasible. If you have a built-in flash it will always be there on the camera, but built-in or on-camera flash isn't the most attractive of looks and may not be the look you are after. And if you're outdoors you can't easily set up a bigger light, and that off-camera Speedlite may be in the cupboard with your tripod!
You could increase your ISO. Increasing the ISO sufficiently will allow you to change your aperture and/or shutter speed settings to get the same exposure.
By increasing your ISO from 100 to 400 you would now be able to shoot at 1/60 second at f/4 and you'd be able to get a decent hand-held shot. But what about that child still moving around? For that you may need to be shooting at at least 1/250 to get close to freezing the child's movement. That's a full 2 stops (1/60 to 1/125 to 1/250).
Fortunately, just like Aperture and Shutter Speed, ISO is calibrated in STOPS too. So you can increase your ISO by 2 stops (400 to 800 to 1600). At 1600 ISO, in these same light conditions, you can shoot at 1/250th at f4, or at 1/125th at f/5.6, or at 1/60th at f/8 and so on and get the same exposure.
The downside of increasing ISO
Its all about the noise. At 100 ISO most cameras will produce pretty noise-free results. Up at 400 ISO some noise may be visible when you look closely, and at 1600 or 3200 ISO all but the very best (expensive?) cameras will create quite a bit of noise.
So its best to shoot at the lowest possible ISO.
But if all else fails use a higher ISO. Better to get the shot then not get the shot.
So back to that triangle
By now you may be starting to understand that to get a particular exposure (let's for the sake of argument call it a 'correct exposure') you have to have a certain amount of light fall on the sensor. This can be done by adjusting the Aperture and/or the Shutter Speed. It can also be done by making the sensor more sensitive so that you need less light to fall on it to achieve the same 'correct' exposure.
This is where the triangle comes in. If you change one point of the triangle, you can change the others. This gives you a range of practical and creative options.
Why does it matter what my shutter speed or aperture is?
Sometimes it doesn't matter very much at all. If you have a subject that isn't moving, and if you're not moving you'll probably be ok with a shutter speed of 1/30 second. Before the days of Image Stabilisation (IS) a good rule of thumb was shoot hand-held at no slower than 1/30, and preferably at 1/60. With IS you may gain a stop or two on that and shoot at 1/20th or 1/15th if you're very careful. But this assumes that you are shooting with a lens of focal length 55mm or less. Longer lenses require faster shutter speeds to avoid camera shake – the longer the lens the more pronounced the effect is, so as a rule of thumb you should shoot at 1/100 for 100mm focal length, 1/200 for 200mm focal length and so on*.
*If your camera does not have a full frame sensor then you should add 50% to these estimates.
But if your subject is moving you will need to shoot at relatively faster shutter speeds unless you are trying to capture the blur of the motion. So for example, a person walking: 1/125, a person running: 1/250, a moving car: 1/1000 (at least).
And if you have a subject that is more of less on one plain (no distinct foreground or background to take into consideration) then it isn't too important what your aperture is. So shooting at f/4, f/8, or f/16 will give you similar results. But let's say you are shooting a person against a busy background and you want to make them sharper against the background and thus simplify the background. Then you'd want to shoot with a wide aperture to get a wider depth of field – say f/4, f2.8, f/2, or f1.2 if your lens can do that.
