Metering a photography scene: How to, and why anyway??



The first thing I do when approaching any scene is meter it. Always. Metering is what you do to tell the camera how the light plays out over a scene.

Why is this important? For any given scene the human eye can capture a huge range of light intensities – in photography this is referred to as f-stops. Depending on the literature you reference, the range over which the human eye is sensitive is 16-20 stops. (That’s huge!)

A typical Digital SLR sensor, on the other hand, can only pick up light over a range of 6-7 stops. When viewing a sunset with your eye (a scene with an f-stop range well beyond 6-7 stops), you’re able to pick up all the detail in the bright sky, as well as the detail in the darker foreground, below the horizon. A camera has a hard time doing that…

The difference in light intensity below and above the horizon is too much for a camera to cope. What’ll happen if the entire scene is metered? Usually, the camera will calculate the exposure for the sky, and the exposure for the darker foreground, and it’ll try to come up with an exposure to capture it all. The result when captured: the sky may be a bit too bright, while the foreground’s too dark. If the sky comes out nicely, the foreground will be really dark.

So how does metering work, and how do you make it work for you? There are typically four different kinds of metering: evaluative, center-weighted, partial, or spot. My own Canon EOS 1Ds Mark II has these, and most often I use partial metering.

In Evaluative metering, the camera breaks the entire scene into rectangular areas (35 for the Canon 20D; 7 columns, 5 rows). It looks at the light levels in each area, compares AlL areas, then calculates the exposure based on all 35 areas. For many comtemporary Digital SLR cameras, and point-and-shoots as well, this method can produce nice exposures.

But imagine this scene: You’re in a mostly dark room, with a shaft of light coming through the window, shining on an object. You’re photographing it from the side, with that brighly lit object in the frame, and lots of dark background all around it.

Photographing that kind of scene with Evaluative metering will result in an average, though, just like in the sunset example above. And this can be undesirable. The fix: switch to Partial metering.

Using Partial metering, you can read the light from the lit object only, and have the camera calcuate the exposure based on that. The resultant picture will have a really dark background, but that’s just fine. In many dramatic lighting situations, you can only have the correct exposure from one object or area. The rest simply gets over or underexposed, which can be perfectly acceptable.

Center-weighted and Spot Metering work the same in the sense that they calcuate the exposure based on a pre-defined area that is smaller than the frame itself. You are telling the camera what light it should pay attention to, and that’s key to rendering the main subject of the shot correctly.

So if you’re heading to a scene with a Digital SLR or an advanced point-and-shoot camera, read the light! It’s all about telling your camera what it should do for the exposure. And how it controls that exposure is through shutter speed and aperture…

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