OK, that’s all the foundations laid, now how do we actually know what exposure to start with, before we start getting concerned with what creative choices we want to make and all of that mullarkey? Well, until such point as you’re able to look at a scene and simply know what numbers to start with (you’ll be surprised how quickly this starts to happen if you use manual mode, and practice) the main way you get started is with the exposure meter built into the camera, and then by working with the histogram on the back of the camera. The exposure meter is a clever device that basically looks at how much light there is in the scene in front of you, and then decides which values of aperture, shutter speed and ISO will give you a “correct” exposure.
If you leave your camera on fully automatic mode (just this once, don’t let me catch you doing it again) and take a shot, the odds are the camera will do a decent job of getting you an OK exposure. Perhaps not brilliant, or what you exactly had in mind, but usable. The way it does this is by measuring how much light is reflected into the camera and onto it’s internal exposure meter, and then choosing from within its own set of options, an exposure that will give you an OK picture. It comes up with these options by chewing over a huge mass of data that has been created by millions of photographers who have gone before you, which has then been assembled, categorised, and calculated by some very clever folk, probably in a big laboratory somewhere in Japan. It tends to bias towards giving you a good shutter speed, to avoid camera shake, and it will usually then adjust the other two factors to suit.
If you shoot in manual mode, rather than auto, you’ll probably find that your camera has arrows or pointers that suggest you shift the exposure you’ve set one way or the other, to arrive at the combination it’s looking for. Take a shot at that suggested setting, and it’ll come out at just the same exposure as an automatic one.
For completeness I ought to talk about different metering modes, but to be honest, I think it may add to any confusion you may already be feeling!
If you’d like a bit more detail, the 3 modes you’ll generally encounter are Evaluative, Centre Weighted, and Spot. The first is, these days, the most common, and can go by different names such as matrix or 3D, and it takes an overall view of the scene, balances all the different areas out, and comes up with what it thinks is an appropriate exposure. More sophisticated versions of evaluative often include distance data taken from the lens, to give priority to parts of the frame that are nearer – they assume you’ve placed the thing you’re bothered about in the foreground and would like it to come out correctly exposed.
Centre weighted has been around for ages, and all this does is give priority to the centre of the frame, as the system assumes you’ll be placing the thing you’re interested in dead centre. The exact balance and shape of the centre pattern will vary from camera to camera, but it will usually be somewhere in a ratio of 60/40 in favour of the centre.
Lastly, spot metering. This allows you to meter from a very small area of the frame, so you can be very specific about which part you want to expose correctly. As long as you use it well it’s very powerful, as it can overcome many of the problems we’re about to talk about with metering systems getting confused by large areas of light or dark in the frame. However, the fact that it’s so precise can often work against it – you’ve only got to be slightly off for the exposure to be all over the place. As with so many other things, the precise nature of how spot metering works will vary from camera to camera, the spots are often tied to focusing points, but check your camera’s manual.
If none of that made any sense, or you don’t see any of those dials on your camera, don’t worry – on the occasions I use the camera’s meter, I use evaluative 99% of the time, and since that’s what most cameras are set for, you’re golden. I still find it faster to take a shot, then check the back of the camera, and change settings if needed, rather than fiddle about with picking a different spot to meter with.