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Figure 6-31: This is an old-fashioned exposure meter. They still serve a useful purpose: When used correctly they will always give you the right exposure recommendations, regardless of how dark or how bright your subject is. Menu Position Menu -->  5 --> Metering Mode What it Does Allows you to select between Multi-Segment, Center-weighted, and Spot Metering Recommended Setting Multi-Segment Metering Super-De-Duper Important Tip: These metering modes evolved from the days of film cameras, when you were shooting blind. Well, now you have live view and you're not shooting blind anymore. So for the vast majority of situations you don't need to know this stuff. Just keep this set to "Multi-segment" and then use the exposure compensation dial until the preview looks right to you, then shoot. I explain the three metering modes below anyway, but if you're impatient, jump to Section 6.23.3 where I show you how much simpler the new way is. |
It’s easiest to understand the evolution of the different metering modes once you understand the history of determining exposure. It all started with the handheld meter like the kind shown in Figure 6-31. They measured the amount of light falling onto your subject, and they were always right.
Handheld exposure meters work on a completely different principle than the ones built into your camera: Handheld exposure meters measure the light that is falling onto your subject. Built-in exposure meters, on the other hand, measure the light that is reflected off of your subject and back into the camera.
Figure 6-32: Average subjects in average scenes reflect back about 18% of the light, and so that’s what your camera tries to create – it is assuming you’re shooting an average subject. As an interesting experiment, when you average together hundreds and thousands of normal snapshots, you will eventually end up with an image that’s about 18% grey. |
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Is this a good idea? Well, yes and no. No because the reflectance of the subject will vary wildly – brides in white dresses, for example, will reflect much more light than, say, a groom with a black tux – and therefore your in-camera meter will give you wildly different values. Yes, because it’s infinitely more convenient to have the camera guess the right exposure than to go over and measure it with a handheld meter. And besides, for average subjects (like pictures of anything except brides and grooms :-) ) the reflected method works reasonably well.
More detail: Once upon a time, an analysis of thousands of different snapshots showed that the average reflectance of an average photographic subject was about 18%. Therefore, when the camera is looking at the reflected light from your subject, it will assume the subject is average: it will assume the light it is seeing represents 18% of how much light is hitting it. It can then infer what the intensity of the original light source must have been, and set the exposure for that value. If it does its job right, all images (when the color information is removed) will look about 18% grey. (See example in Figure 6-32.)
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Figure 6-33: Examples of where the exposure meter tries to make things look 18% grey (and how using exposure compensation can fix it). These errors would never have happened if a handheld exposure meter were being used! Yet another example of where we trade off convenience for accuracy. |
The fact that the camera tries to make things look 18% grey explains a great deal of the exposure mistakes you might see your (or any other) camera make. (Have a look at Figure 6-33 for some startling examples.)
And so, this basic principle of assuming that the average of all scenes should be rendered as 18% grey persists to this day. And although it’s far from perfect, on average most point-and-shooters have been very happy with its decisions. People in-the-know (and readers of this book) tend to be happy making the occasional override using their Exposure Compensation function when necessary, in exchange for the convenience of NOT having to use the handheld exposure meters.