Thus far we’ve taken for granted that our shot looks OK, and that we’ve got the “right” exposure – all we’ve done is play with the variables. We’re just coming to how we arrived at those numbers in the next section, but first let’s think about what happens in practical exposure terms, how much light there is, and what happens if we get the exposure “wrong”. For continuity, let’s go back to an earlier image, and see what happens if we just change one variable, in this case the shutter speed:
That doesn’t look great, does it? When the image looks too dark it’s known as under exposure, and when it’s too bright, it’s over exposure. Generally speaking we want to avoid either of these, although a little of each can be used for creative purposes.
Practical Exposure and the amount of light.
This is the perfect time to bring up the elephant in the room – how much light is there? So far I’ve been chucking all these numbers about in a very abstract way, but overlooking one very vital fact, which is that the correct exposure (or combination of exposure factors, to be more accurate) is based on how much light there is in the scene we’re trying to photograph in the first place. It may well be that with the amount of light we’ve got, the creative aim we’re heading for isn’t possible.
One of the simplest ways to think about this is imagine how much light there is outside on a bright, sunny day, then imagine how much light there is indoors, after dark, with just one small lamp on. Now, our eyes adjust to these differences quite quickly (but not instantly, as we’ve all experienced at one time or other) but of course, a camera doesn’t. If your camera is in automatic mode (boo, hiss, bad) then it will adjust its aperture, shutter speed and ISO to come up with an exposure of some sort. Of course, in manual mode (hooray!) this won’t happen, and if you move from bright sun to indoors and keep the same settings, you’ll be several stops underexposed.
Do a bit of mental arithmetic, and you’ll realise there was 5 stops difference in the total amount of light between these two shots and situations. 2 stops in shutter speed between 1/30s and 1/125s, and 3 stops between the aperture of f4 and f11. The amount of light you’ve got to play with will affect your choice of the three exposure variables. In both these cases, the exposure was “correct”, in that it does the job it’s supposed to do, but the numbers are very different.
The amount of light we’ve got to work with is obviously going to affect our choice of exposure combination to some extent. In the example of the darkened room, we’d struggle to get a fast enough shutter speed to freeze any fast movement, certainly if we also need to have a medium to small aperture to get a decent amount of depth of field. We’d need to turn the ISO up quite high to get anywhere near the numbers we would want from the other 2 factors, and that of course would increase the noise in the image. Before you assume that therefore it’s always better to have loads of light, imagine being outside on a very bright sunny day and wanting to shoot with a very large aperture to get that lovely, shallow depth of field. There may be so much light about that you can’t get a fast enough shutter speed without the image ending up over-exposed, even if you dial down the ISO as low as it will go.
In numerical terms, let’s say we’re indoors in a sports hall, lit by artificial light, and we’re trying to freeze the motion of people playing basketball. We know from experience that we need a shutter speed of 1/250s or faster, and the widest aperture on our lens is f2.8. At ISO 100 this exposure is coming out way too dark. We know we haven’t got any more aperture to play with, and if we slow down the shutter speed to let more light in the players will blur. So we can either add more light (using something like a flash, or a whole truck full of movie lights…) or we can increase the ISO until we’ve got the right balance.
Let’s imagine a situation with too much light. We’re outside on a really bright day, and trying to take a portrait of a friend, and we want a really shallow (small) depth of field, so we’re shooting wide open on the same lens at f2.8. Even at ISO 100 (which is as low as our hypothetical camera goes) we’re still over-exposed at the fastest shutter speed on the camera of 1/4000s. Now we need less light, which could mean moving into the shade, or reducing the light coming through the lens by adding a neutral density filter to the front of it – think of a neutral density filter as a pair of sunglasses, cutting the amount of light coming through. Now stop thinking about filters, as they’ll only confuse you for now!
If you’ve been following me so far, and have had your DSLR or mirrorless camera in front of you the whole time, you’ve probably noticed that I seem to be missing out lots of numbers that you can see on your camera. Where’s f3.5? Where’s ISO 250? What about 1/40s shutter speed?
You’ll have to forgive me, but in an attempt to keep things simple when we got started, I skated over the fact that on most modern cameras you can set all 3 values to ranges between the main numbers I’ve listed. Generally speaking you can choose numbers in either 1/2 or 1/3 stop values, and this allows you to be much more precise in your exposure – as you’ve seen, a whole stop can be quite a big shift. Personally I have all my cameras set up so I can adjust in 1/3 of a stop. It’s precise, but not so precise that it becomes cumbersome and slow. Back in the days when I worked as a photographer’s assistant, one of the photographers I worked for demanded 1/10th stop accuracy in exposure. He wasn’t much fun to be around, and I insist that no human being can really discern 1/10th stop difference, and certainly not set one on an old fashioned aperture ring!