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Saturday, December 1, 2007

Digital Exposure

What does good exposure give you? And can you trust the histogram?

Here are tips to help use the histogram.

No gaps on the right.
A gap of data on the right shows as a flat line. You see the mountains and valleys to the left, then they drop to nothing. This means underexposure and underutilization of the sensor.

Expose to the right in moderation.
If you have a scene with a lot of darkness in it, avoid putting all of the dark values all the way to the left. Move them out from that side by increasing exposure (a slight gap on the left is fine as long as bright tones aren’t crammed against the right side).

Watch, but don’t be governed by exposure warnings.
Some underexpose images to avoid those blinking highlights. Don’t do that. That makes dark areas have problems with noise and chroma. If detail in a bright area is important, reduce exposure until the blinking highlights quit there, but no further. If detail in a bright area is unimportant (such as a bit of bright sky), ignore the blinking warning and let that area get overexposed as long as the rest of the image is okay.

Watch for clipping of highlights.
You don’t want the graph to be chopped off or clipped on the right. That clipping of highlights means there's no detail in some very bright areas or you’ll have a heck of a time trying to pull detail out of those areas.

Let low-contrast scenes fill the middle of the histogram.
Foggy scenes or haze on a landscape give a histogram that doesn’t go completely from left to right. There likely will be a hill much smaller than the space available. Avoid exposure that puts the hill toward the left side. You can push the exposure so the hill goes to the right, but keep it more or less in the middle.

You don’t need to check the histogram for every shot—only occasionally to be sure you're getting good exposures and more often when conditions are challenging.


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