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Using the Histogram

When reviewing images, preferably on-camera while you still have the chance to re-shoot your subject, you should be attentive to two things:
  1. Is the composition in the ballpark (remember, you can always crop later)
  2. Is the histogram ok?
The histogram is the technical bit here, I'll try to explain it here in brief.

This is a histogram. This one happens to be from the GIMP, but whether you're looking at a histogram on your digital camera, in Photoshop, in the GIMP, or in another program, they are all the same. The histogram shows for all tones (from dark on the left to light on the right) how many pixels have that tone in the image. The image above is farily typical with a relatively even-distributed histogram from dark to light - all tones are represented.

If you would overexpose your image, your histogram would look like this: too many highlights, no darker tones, and you can guess that you have lost information: light tones have become lighter than the camera can handle and have been stored as completely white. You should re-shoot with a smaller aperture or a shorter shutter time.

On the other hand, this is how under-exposure looks like: no highlights, the histogram is shifted to the left, and lots of information from the image will be missing because the chip in the camera didn't catch light. Re-shoot with a larger aperture or a longer shutter time.

There's a technical argument around histograms that basically says that there is more information in the higher tones of the image in a digital camera - so, if possible, you should "expose to the right", meaning that your histogram should be shifted a bit to the right, but not too much. Of course, the histogram shown on your camera is an average of all colors - it might be that even though the average color histogram looks fine, a single color (red, green or blue) histogram would show over- or underexposure. Therefore, "expose to the right", but realize that over-exposure of individual colors is possible - better be safe than sorry so take an extra shot that is 2/3rd or a full stop less exposed, so that when you get home and evaluate the individual color channel histograms, you have something to fall-back to when your optimal exposure turns out to be overexposed for a single color after all. Lots of digital cameras have auto-exposure bracketing, and I use it a lot (bracketing 2/3rd of a stop to both sides).

When shooting in auto mode, try overexposing 1/3 stop to shift your histogram just that little safe bit to the right. Of course, experimentation is key - the great thing about digital is that experimentation is all for free!

Sometimes there's more in the image than your camera can capture - a digital camera will capture around 5 stops of dynamic range, and if the dynamic range of your subject is larger than that, you will have to make a choice to let the highlights or the dark tones fall off. The Canon G5 has a built-in Neutral Density filter that will compress contrast, so that is another option (external, 'classical' ND filters, of course, will work just as fine on digital cameras).

Keeping in mind that a histogram on your camera usually represents 5 stops worth of dynamic range helps you calculating the correct exposure when correcting a histogram: for example, the underexposed histogram above shows that more than a fifth of the highlights aren't used, therefore increasing the exposure by a full stop will move the histogram 20% to the right, where it still 'fits'. More than that (1 1/3rd stop) and you risk overexposure, less than that and you're not exposing "to the right" enough.

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