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:
- Is the composition in the ballpark (remember, you can always crop later)
- 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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