Why Matrix Metering Makes Learning to Meter More Difficult
I wrote last week about leaving your gray card at home when metering in the great outdoors. Manual metering isn't really that difficult, but it can seem that way at first. One of the main reasons many photographers resist learning is that they can often get away without doing so thanks to Matrix or Evaluative metering. But such systems aren't perfect, and they can get in the way of your taking full control.
Nikon advertises that their matrix metering system "takes into account the scene's contrast and brightness" using "special exposure-evaluation algorithms, optimized for digital imaging, that detect highlight areas. The meter then accesses a database of over 30,000 actual images to determine the best exposure for the scene." Their marketing material has included statements similar to this for years. Even back when computers cost a fortune and photography relied on rolls of film for storage Nikon claimed their cameras in some sense contained such image databases. While it is conceivable that something like this might be possible today, this many actual images could never have been stored inside 1980-vintage cameras.
Instead, what is more likely the case is that information about that many images lives inside the matrix metering chip. Evaluative meters look at the world as being comprised of a number of segments. Early Nikon matrix meters carved a scene up into just five metering segments while more modern bodies include meters with either 420 or 1,005 segments. I can believe that the meter could determine a value for each segment of a scene and compare it against a database of meter readings to find one that fit, but no camera meter actually has a library of actual images inside.
Because it's a trade secret, nobody really knows how Nikon's matrix meter works. The same could be said for Canon's equivalent or course. And it's precisely this inability to understand what the darned thing actually does that can make it difficult to fully master. To the best of its ability, it will produce what it considers a well exposed image, but there's no way for it to know what you really want. Is it your image, or is it merely the result of what the matrix meter thinks of the scene?
If you use matrix metering enough, you will come to understand at least some of what it can and can't do of course. If you're shooting something similar to a shot you've made before you might recall how well the matrix meter did last time. If your sunset images typically come out over or underexposed, you might learn to compensate with the exposure compensation control. You might learn that other types of images require compensation as well to varying degrees. The basic problem with matrix metering is that you can never really know how heavily it weights a bright reading from segments in one part of the frame any more than you can what it will make of darker segments elsewhere in the frame. Remember, it's a trade secret.
The more you shoot the more familiar you would naturally become with the meter, but the best you can hope for is to learn to help it make better decisions rather than have it help you make decisions. A matrix or evaluative meter has a mind of its own. To even begin to take control of it you have to first wrestle with it for long enough to learn its strengths and weaknesses to gain the upper hand. And when you upgrade to a new camera model at least some of what you knew about the old one will have to be relearned. Nikon or Canon will surely have changed things at least a bit since you bought your last camera.
By contrast, manually spot metering a scene to determine exposure is a skill that, once learned, can be employed with any camera, of any vintage. So long as the meter reads accurately, you can rely on what it is telling you about the spot you are pointing it at. Nothing is secret. A spot meter will tell you everything it knows.
Granted, if you point a spot meter at the wrong spot, or too few spots to fully come to a decision as to what aperture and shutter speed to use, you'll end up with an image that doesn't look the way you thought it would. Granted too that spot metering manually will surely take more time than letting the camera's matrix meter do everything for you on full programmed automatic. But when you can live within these parameters the exposure you end up with is what you make it. You're the one in control, not the camera.
The ability to review digital images and their histogram after shooting them does mitigate things a bit and I do sometimes use matrix when the light is changing quickly or the subject is moving too fast. But matrix metering isn't my preferred metering system for most situations. I like being able to create images exposed the way I want to, not the way my camera might decide to.