We are almost done with exposure. We have seen already what it is, how to measure it via the histogram and how to control it via shutter speed, aperture and ISO. The last piece of the puzzle is understanding how the camera decides what constitutes a good exposure – how it meters the scene.
In its most basic form, the meter of your camera will measure light coming from the subject through the lens, then use that information to set exposure parameters which would result in an average exposure of 18% grey – that is, the exposure is the same than if you had photographed an even bright gray light source. You are then free to modify either the total exposure (via exposure compensation or manual mode) or the individual parameters.
On most modern cameras, three different meter modes of greater complexity than the 18% grey can be found. In order of usefulness, they are:
For instance, snow scenes are notably tricky to expose, as the camera may panic, thinking all this whiteness is awful overexposure which must be brought down to a more reasonable bright grey. Some modern cameras are now smart enough to recognize that it is indeed snow and should be left very bright, while others still need a manual one or two stops of overexposure. The only way of knowing how smart your particular camera is is to try it in a variety of situations.
So in a nutshell, it is usually safe to stay in multi-zone metering for most of your shooting, with two caveats: the camera can make mistakes in complicated scenes and shouldn’t be trusted completely, and spot metering is useful in high contrast scenes, especially when the subject you want to expose for is small in the frame.
There is a button found on most cameras which tends to remain fairly obscure (some would say scary): AE-L/AF-L. What it does is lock either exposure, focus or both (it can be decided in the menus). If you set it to exposure lock, it will be useful when you shoot in spot mode: put your subject in the centre, press the shutter halfway to meter, then press AE-L to lock the exposure parameters, then recompose to put your subject where you really want it – as we will see later in this course, it is very boring to position a subject dead in the centre of the frame. AE-L tends to be less useful with multi-zone metering.
In today’s assignment, you will have a bit more freedom than usual, as it will depend heavily on the subjects you find. Try to find a subject difficult to expose, either because it has a lot of contrast or because it has large parts intentionally darker or brighter than 18% grey. Try to catch your multi-zone meter making a mistake, and see if you can reproduce this with another similar subject.
Find a small, bright subject in a dark environment – it could simply be a room with lights shut and a headlamp shining on a piece of paper, and try to expose properly with multi-zone meter. Now do the same in spot mode. For bonus points, position the subject well off-centre.