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.
