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Every digital
camera user needs to understand the exposure tools their camera provides.
One of those is the histogram. Unfortunately, camera manuals don't provide
much help in evaluating histograms, and not a lot has been written about
them. Moreover, I haven't been able to find many illustrated examples.
The purpose of this page is to help fill in that gap in information.
But first,
I need to establish a few "basics":
When viewing
on the camera, the primary histogram faults to look for, in order of precedence
are:
- Histogram
extends off right edge.
Result: loss of highlight detail.
Worst case: a tall line at the right edge of the histogram.
- Histogram
extends off the left edge.
Result: somewhat unrecoverable shadow detail (exposure changes with
NEFs in Bibble or Capture might pull some back, but in JPEGs shadow
detail is not recoverable).
Worst case: a heap that peaks at the left edge and NO exposure to
the right of center.
- Histogram
extends from side to side, but there are almost no values at the extremes
(e.g., the entire right or left portion of the histogram is nothing
more than a flat line).
Result: somewhat problematic image in printing without excessive
Photoshop manipulation (shadow and highlight detail may be tough to
hold together--you may have to pick one or the other).
Worst case: small lines at either or both edges indicate a scene
whose exposure is outside your camera's ability to record.
- Histogram
is peaked in one narrow area, and doesn't extend towards edges.
Result: you'll be using Curves to pull out the detail in Photoshop,
and the more narrow the peak is, the tougher that task will be.
Worst case: the peak is in the lower 1/4 or upper 1/4 of the scale,
which would indicate under- and overexposure, respectively.
Okay, we're
ready to look at some real histograms:
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So
let's start with a simple histogram. If you've ever taken a well-exposed
picture of a gray card, you should recognize this histogram. There's
one well defined peak because virtually all the values on the card
are the same. The peak is just left of center because a gray card
is 18% gray, not 12% (see my discourse
on that subject for more). I've got a few straggler peaks to the
left of the main peak because I wasn't particularly careful about
keeping a bit of shadow off the card.
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Here's
a slightly more difficult histogram to recognize, but one that many
of you should have seen before. The giveaway is that there are a number
of distinct peaks. Peaks always indicate the presence of a large number
of pixels that are all the same value. Again, I've got a bit of shadow
on my subject, so the peaks aren't as distinct as they sometimes are,
but this is a histogram of a perfectly exposed Macbeth ColorChecker
card.
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So
here's our first puzzler. What's this a histogram of? It's darn
near perfectly exposed (the Highlights setting on my D1x showed
no flashing pixels indicating no loss of highlight detail).
The bulk of the pixels are just to left of center, which I consider
the D1x's "sweet spot" for exposure, as it gives me the
most flexibility for post-exposure adjustment. While there are pixels
to the right of center, they're spread out across a wide range and
there are more of them the further you get from white. This means
I should be able to easily control and manipulate highlight detail.
Likewise, I don't have any pixels that fell off into black at the
left. In other words, I've managed to keep every pixel of
the image in the dynamic range of the camera. (Hint: I was using
both a polarizer and a 2-stop graduated neutral density filter to
pull the bright areas down.)
This
is a reasonably well-controlled landscape shot (in this case, taken
from the viewing area above the Kennedy memorial at the Arlington
National Cemetary. Generally, I try to keep a histogram from falling
off either edge, and in this case, I was successful. (Note that
the Unsharp Mask and JPEG compression has taken some of the brighter
areas to absolute white in the example you see at left.)
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Here
are a pair of histograms (pulled from Digital Pro) taken within
moments of one another. I changed only one thing between shots (hint:
I added something). Can you guess what it was and why I added it?
I added
a graduated neutral density filter. (The picture was of Bryce Canyon
near sunset, but unfortunately, I've seen to have lost the example
photo in many gigabytes of images on my network at the moment.)
The hump at the far right was basically the sunset sky (the unlit
hoodoos below the ridge were the left hump). The ND filter brought
all the sky values down into the same general exposure as the landscape,
which allowed me to use contrast control to spread the histogram.
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Want More?
One
of the reasons I started the D1 Report is
to discuss at length (and with high resolution examples) some of the issues
that occur when shooting with a D1. Get your subscription today!
Also, my
Complete Guide to the Nikon D1, D1h, &
D1x includes additional information on histograms and exposure
setting.
But if you
want hands-on training, nothing beats taking one of my three-day
workshops. More often than not, these workshops are filled with D1
users and become an all-digital workshop. But since I only take three
students at a time, you'll get plenty of one-on-one time, and plenty of
histogram interpretation from me.
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