correctly exposing with RGB histogram / how to read rgb histogram

Started Jul 10, 2007 | Discussions thread
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jfriend00 Forum Pro • Posts: 11,313
Re: correctly exposing with RGB histogram / how to read rgb histogram

First of all, you cannot directly determine whether your image is properly exposed with the histogram. What the histogram should look like for a perfectly exposed photo is very dependent upon the particular image you are shooting. Properly exposed images of a black cat or a snow skiier in the sun will both have very different histograms. But, you can tell a lot from the histogram. Here are some things you can tell from the main histogram:

o If there is clipping on the right side, then highlights are blown.
o If there is clipping on the left side, then shadows are plugged.

o If the histogram is narrow and doesn't fill the whole range, then the image is a low contrast image.

o If the histogram spans the whole width with meaningful volume all the way up to both ends, then the image is probably a high dynamic range image that exceeds the capabilities of the sensor to capture both highlight and shadow detail in one exposure.

o A histogram with nothing on the right side has none of the upper quarter tones. If your image is supposed to have the full tonal range, then it's significantly underexposed. But, it might be very common on a dull, dreary, heavy overcast day to take a shot where the natural and correct exposure would not fill the tonal range or the right wide of the histogram. This would be a naturally occurring, low contrast image.

The R, G and B histograms are just the histograms for the individual R, G and B channels as recorded from the sensor. Since that's what the sensor actually records, the composite histogram is really just a weighted average of the R, G and B histograms. Here are some things you can learn from the individual channel histograms:

o If any individual color histogram is clipped on the right side, then highlights in that channel are blown.

o If any individual color histogram is clipped on the left side, then shadow details are plugged.

o If a large bright object in your picture is supposed to be neutral (white or gray with no color), then you should see a spike in the R, G and B histogram all in the same place. If the spikes do not line up (and the spike is supposed to be a neutral object), then your white balance is off.

In practice, the main thing I use the R, G and B histograms for is to make sure that I'm not clipping the highlights of any single color channel. On many digital sensors, it is fairly easy to clip the red channel and not necessarily be able to see that from the overall luminosity histogram (because that's a weighted average of all three color channels). You can also sometimes get a nice quick check on white balance if you have a bright, large neutral reference in a test shot.
--
John
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