Learn To Read Your Camera's Histograms And Get The Best Exposures
Possible For Your Subject
Text And Photography By Rob Sheppard
Modern digital cameras
do an impressive job in getting an acceptable exposure for most
images. There’s a difference between an acceptable exposure
and an exceptional exposure, however. The best exposures make
maximum use of the capabilities of the camera sensor and internal
camera processing while causing only a few problems when printing
or working on images in the computer.
While it’s
true that image-processing programs do allow some “fixing”
of a photo in the computer, you’re wise to get the best
possible image captured to the memory card in the first place.
This results in an image with clearer details, better tonal
definition in highlights and shadows, improved color and fewer
noise issues.
There’s a serious misconception that with RAW capture,
shooting the image right from the start is no longer important.
Not true! The image sensor is only capable of handling a certain
range of brightness. Outside of that range, you have problems.
RAW files can help with challenging exposures, but even that
format works best when the exposures also are at their best.
One way to ensure that your exposures are accurate is to use
the histogram display, if your camera has one. More and more
cameras are including this valuable technology, including so-called
point-and-shoots priced at less than $500. You don’t need
to use it for every image, but if you check it when the light
is tricky and for critical photos, the histogram can give you
important information about an image’s exposure.
The histogram sometimes looks a little intimidating. It was
never part of film photography, and it certainly doesn’t
look very creative. It’s a graph based on mathematics,
but to use it, you don’t have to know anything about math.
This article will give you an overview of what to look for in
your histogram and offer some specific photographic examples
showing both an image and its histogram.
Histogram Basics
While the histogram looks rather technical at first, the basics
of reading it are fairly simple. Camera manufacturers haven’t
always made finding the feature in the menus that easy, so check
your camera’s manual to see how to turn it on, if your
model includes it.
With digital SLRs and many digital cameras, you have to look
at the histogram after you take the shot. With some digital
cameras, you can turn on the histogram for live viewing as you
prepare to take the picture, which can be helpful in adjusting
exposure on the fly.
The histogram is a chart that looks like a hill, mountain range
or multiple hills, but the key elements are the slopes at the
left and right. Some histograms are solid graphs, while some
just show a line where the tops of the points are—both
are identical in use, only different displays.
To read exposure, begin by looking at the left and right parts
of the chart. The left shows the darkest parts of the scene
and the right displays the highlights. The entire width of the
graph represents the total range of tones that your sensor can
capture. Anything beyond the left edge is pure black, outside
of the range of the sensor, so nothing is recorded in those
shadows. Anything beyond the right edge is pure white, and again,
outside of the range of the sensor.
A satisfactory exposure will have a full range of tones from
left to right appropriate to the scene. There’s no “right”
shape to the histogram, since that will change depending on
the types of tonalities in your scene. The key, though, is to
be sure no exposure information is clipped at the right or left
(the graph is abruptly cut off), which represents the exposure
level where detail is gone. A cutoff or clipped histogram means
details aren’t being captured at all.
That’s not necessarily wrong, however. Some scenes simply
have too great a contrast range for your sensor, which will
result in clipping at either the left or right (or both) because
it’s impossible for your camera to do anything else. You
may have to decide what’s most important for your scene—highlights
or shadows—and be sure the histogram isn’t clipped
at either end. Bright specular highlights from reflections of
glaring lights probably should be pure white, for example. Or
the darkness of a deep shadow might look just fine as black.
Underexposure will show up with little or no data on the right
side of the chart, while the rest of the data is skewed to the
left. Often, the hill at the left will be cut off sharply at
the far left edge. This can cause two major problems. The first
is obvious—less detail in the shadows. The second appears
when you try to adjust the image—noise. Underexposed, dark
areas often will pick up annoying and distracting noise.
The correction is fairly easy: add exposure. If you’re
on automatic mode, use the + (plus) part of exposure compensation
to bump up exposure.
Overexposure demonstrates the opposite. You’ll find little
or no data on the left side of the chart compared with the right,
and the right side often will be cut off sharply. That cutoff
is a real concern, since those areas of exposure beyond that
level will be washed out, appearing blank in the photograph.
The correction: subtract exposure. If you’re on automatic
mode, use the – (minus) part of exposure compensation to
lower exposure.
A low-contrast scene shows something entirely different. Typically,
it will show a histogram with an entire hill of data well within
the left and right borders. No tonal values will be seen near
either the left or right side. This can be a challenging histogram
because often it will need to be stretched in your image-processing
program. That stretching can cause problems with tonalities
banding because not enough data may be present to support all
the tones.
Some low-contrast scenes should be kept low contrast, such as
foggy skies. Other scenes need correction, however. This is
a good scenario for RAW capture, since it will allow stretching
of tones without damage to the image. On many cameras, you can
choose different contrast settings for the conditions, for example,
setting a custom parameter on your camera just for cloudy days
that increases contrast and saturation.
Overall, you want your photo to display a histogram that slopes
down as close as possible to the most important brightness values
of your photograph. If the highlights are critical, then the
slope must end at or before the right side. If the shadows are
essential, then the slope must favor the left side. This can
be decisive in getting an image with proper data going into
your computer.
There are several ways of making these adjustments. The easiest
way to deal with a scene is to become very familiar with your
camera’s ± exposure compensation control. Dial it
down (minus, for less exposure) when the histogram is bumping
the right side of its area; dial it up (plus, for more exposure)
when the histogram is too far to the left.
There are no absolute rules for using a histogram. Every scene
is different. You’ll need to interpret the histogram as
it relates to the scene in front of you and what you believe
is a good way of seeing the scene. It’s worth experimenting,
since there’s no cost to taking extra shots in digital.
Try different exposures and see what they look like on the histogram
to learn how this function reads various scenes.
