Digital Focus: More Exposure Tricks

Feature: Locking in the Right Exposure

My eleven-year-old daughter told me a joke recently: A skeleton walks into a bar. It goes up to the bartender and says, "Gimme a beer ... and a mop."

What I like about that joke is the fact that the skeleton knew that, given its physical limitations, it was going to have to make some compromises in its quest for something to drink. And that's a lot like the way we take pictures with a digital camera--everything is a compromise. If we want to capture a bright blue sky, for instance, odds are good that the exposure elsewhere in the picture will be less than perfect. That's why many digital cameras have a feature called exposure lock. Mastering your camera's exposure lock is a handy way to ensure your pictures make the right compromises, and your pictures come out the way you want.

Finding Your Exposure Lock

The exposure lock works pretty much the way it sounds: When activated, it locks the camera's exposure settings so you can compose the picture any way you like and press the shutter release at your leisure. So if you want your landscape photo to have a perfectly blue sky, lock the exposure in the heavens, then compose and snap your picture.

But not all cameras include exposure lock. And of those that do, there's no standard way to activate it. Check your camera's user guide for details. The most common exposure lock system is activated by a button on the top or back of the camera, usually right under the spot where your right thumb would rest. The moment that you depress the button, the camera measures the available light and sets the exposure. And as long as you hold the button down, the exposure won't change. That's just an exposure lock--the focus isn't locked, of course, until you actually press the shutter release.

I've also seen cameras that lock the exposure not with a separate button, but by depressing the shutter release partway. With this sort of camera, you aim at the point in the picture you want to emphasize with perfect exposure and apply slight pressure to the shutter button. After a moment, the focus and exposure levels both lock, and you can then recompose the shot to taste. Press down the rest of the way to take the picture and remove the lock.

Exposing for the Bright Bits

Whichever method you need to use with your camera, keep in mind that while this is a great way to make sure that a specific part of the scene is properly exposed, other parts of the image may suffer--especially if the lighting is radically different throughout the image. Consider the sky example, for instance. Ordinarily, digital cameras tend to overexpose the sky because they're trying to retain the detail in the darker ground elements of a picture. The result is that your sky loses color: It's bleached white by overexposure.

To retain the blue sky, point the camera up at it and lock the exposure, then recompose the shot to include the ground-level details. The camera's metering system "shuts down" to a small aperture and high shutter speed to accommodate all the bright light streaming in from the sky. So while the sky comes out looking great, with rich, attractive color, the darker ground will end up being underexposed. Is that okay? It's really up to you--there's no such thing as a proper exposure in photography.

Overexposing With the Exposure Lock

Exactly the opposite happens in the shadows. Usually, digital cameras render shadows as deep, detail-less black blotches, because there isn't enough light to see what's really there. To fix this, point the camera into the shadows and lock the exposure. When you recompose the shot to include the picture's much wider exposure latitude, the brighter bits will be overexposed because the camera is using a slower shutter speed and a larger aperture to pull more detail out of the shadows.

It's Your Call

A lot of the time, you may not need to use the exposure lock at all. The exposure meter averages the light throughout the picture pretty well, giving you a perfectly acceptable shot most of the time. For more info about exposure meters, read "Master Exposure Metering Modes."

But there are occasions when the lighting is simply too tricky: There's far too much contrast in the scene for the camera to figure out the exposure on its own. It's times like that when you need to step in, decide which aspects of the scene are most important to you, and rely on the exposure lock button to capture the scene you see in your mind's eye. Stay tuned for next week's column, in which I'll reveal a technique for getting perfect exposure even in very high contrast situations.

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