How Histograms Run the Gamut
The signature look of Ansel Adams’s photographs is the rich, dimensional details that fill every inch of his prints. Sunlight gleams off
rivers without washing out the water itself. Shadows give mountains density and form without hiding the trees, waterfalls, and outjuts
that fill the shadows. Some of it, even Adams admitted, was luck, having a loaded camera pointing at the right spot when bulging,
dark clouds decide to part long enough for the light to extend a finger to stroke the landscape. The rest of it was hard work and
genius. Adams spent as much time polishing his own technique of developing film and perfecting his print-making as he did
shooting large-format pictures in the field. Few of us have the patience, knowledge, or equipment to follow Adams’s lead. But digital
photographers have something almost as good: It’s the histogram. A histogram tells you instantly whether your exposure is simply
correct for much of a picture or whether light and shadow are perfectly balanced for the entire photo.Ansel Adams’s “Tetons and Snake River”
On many digital cameras, the LCD screen that functions as viewfinder and private
screening room also provides a graph—a histogram—that supplies more information
than a light meter can give you. An exposure deemed acceptable by a light meter,
when analyzed in a histogram, can actually turn out to consist of clumps of darkness
and light that smother and burn away the details that add visual richness to a photo.1
While the photographer is composing a shot,
the digital camera’s processor tallies up how
many of the imaging sensor’s pixels fall into
each of 256 levels of brightness. The
processor graphs the levels to 256 columns—
or fewer for expediency—but still enough to
do the job. The column farthest to the left
represents 100% black. The column farthest to
the right represents 100% white. The columns between them represent a steady progres-
sion of brightness levels, beginning on the left with a level virtually indistinguishable from
black itself and proceeding to the right, with each column slightly lighter than the one to
the left of it. The height of each of them is proportionate to the number of pixels of each
shade found in the frame being shot. On some cameras, the histogram reacts in real time
as you adjust the exposure so you’ll know before you press the shutter how close you’re
coming to an acceptable dynamic range.2
Dynamic range is a ratio of extremes.
It exists in music as well as photography.
A piano has a wide range based on the
ratio of the highest note it creates to the
lowest. Cymbals and tubas both have
small ranges because there’s little differ-
ence between their highest and lowest
notes. In photography, dynamic range
is the ratio between the darkest pixel
and the lightest. When dynamic range is
narrow, the result is a photo such as this
one. The red dots under the bridge show
where the colors aren’t covered by the
sensor’sgamut—the range of colors the
camera can capture. For values outside
its gamut, the camera substitutes the
nearest value it can provide. In this
photo, the gamut runs to blacks too soon
to capture the shadow details beneath a
bridge. It’s like an orchestra that lacks a
bass section.3
82 PART 2 HOW DIGITAL CAMERAS CAPTURE IMAGES