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Photoxels brings you
fact sheets on the best digital cameras.
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More and more consumer
digital cameras are now including a histogram display either during the Record
mode or in the Playback mode. This tutorial explains just what the histogram
is and how you can use it to ensure a correctly exposed picture.
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What The Histogram Is
The histogram is simply a graph that allows you to judge the brightness of
an image. You can think of the area under the graph as comprising all the pixels
in your captured digital image. The left side of the histogram depicts how many
"dark" pixels you have captured; the right side, how many "bright"
pixels you have captured.
Let's look at the histograms of three pictures I took to illustrate how to
read and use a histogram:
Too Dark
Just Right
Too Bright
The left histogram indicates you have most of your pixels toward the dark.
The pixels also touch the left edge, indicating underexposure [note
1]. If Goldilocks had stumbled onto this histogram while taking a picture,
you can hear her saying something to the effect: "This picture is too
dark."
The right histogram indicates you have most of your pixels toward the highlight.
The pixels also touch the right edge, indicating overexposure. Looking at this
histogram, Goldilocks would wail something to the effect: "This picture
is too bright."
The middle histogram depicts a correctly exposed picture with the pixels mostly
in the middle, i.e. neither too dark nor too bright. (Goldilocks: "This
picture is just right.") Notice a few pixels touch the dark edge, so
we should also expect a few very dark spots in the picture.
Perhaps the best way to understand the histogram is to look at the sample shots
that go with the above three histograms.
Too Dark
Shutter Speed 1/180 sec., Aperture F2.8, ISO 50
Aim your camera out the window toward a street light at night or at a scene
mostly in deep shadows, like the one above. Look at the histogram produced:
At a glance, we can see that most of the pixels tend toward the dark (D) side.
The pixels also touch the left edge, so we know that there are underexposed
("too dark" -- thank you, Goldilocks) areas in our picture.
To obtain a correct exposure, we can do a couple of things, taking the above
picture as example:
In Program AE mode, dial in a positive exposure compensation in +1/3EV steps.
(For the above picture, we'll dial in +1/3EV.)
In Manual mode, dial in a slower shutter speed, while retaining the same
aperture. (For the above picture, the shutter speed used was 1/180 sec., so
we'll dial in a slower shutter speed at 1/125 sec.)
In Manual mode, dial in a wider aperture, while retaining the same shutter
speed. (For the above picture, an aperture of F2.8 was used, so we'll set
a wider aperture of F2.0.)
Depending on your camera, you may or may not be able to do one or more of the
above. For example, most consumer digital cameras have a maximum aperture of
F2.8, so we would not be able to try the third option.
What we are doing is telling the camera to ignore its light meter and allow
the light to register on the image sensor for a while longer. We want the histogram
to shift right. Check the new histogram produced and adjust again if the histogram
indicates the picture is still too dark.
Too Bright
Shutter Speed 1/56 sec. Aperture F3.2, +1EV, ISO 200
On a bright sunny day, aim your camera at a landscape photo making sure to
include much of the bright sky or areas bathed in bright lights or sunlight,
as in the picture above. Here, I am exaggerating with a way overexposed shot
(I used +1EV to force an overexposure) to illustrate the point. Look at the
histogram produced:
At a glance, we can see that most of the pixels tend toward the highlight (H)
side. The pixels also touch the right edge, so we know that there are overexposed
("too bright" -- thank you again, Goldilocks) areas in our
picture. To obtain a correct exposure, we can do a couple of things, taking
the above picture as example:
In Program AE mode, dial in a negative exposure compensation in -1/3EV steps.
(For the above picture, we'll dial in -1EV since I gave it a +1EV to force
overexposure.)
In Manual mode, dial in a faster shutter speed, while retaining the same
aperture. (For the above picture, the shutter speed was 1/56 sec., so we'll
dial in a faster shutter speed at 1/125 sec.)
In Manual mode, dial in a narrower aperture, while retaining the same shutter
speed. (For the above picture, the aperture used was F3.2, so we'll set a
narrower aperture of F5.6.)
Depending on your camera, you may or may not be able to do one or more of the
above. Since the exposure settings chosen by the camera for the above picture
are well within the limits of our digital camera, we have quite a bit of leeway
here to play with the exposure settings.
What we are doing is telling the camera to ignore its light meter and allow
the light to register on the image sensor for a shorter time. We want the histogram
to shift left. Check the new histogram produced and adjust again if the histogram
indicates the picture is still too bright.
Balanced Exposure
Shutter Speed 1/60 sec., Aperture F2.8, ISO 50 with Flash
This is an indoors shot using flash. Without flash, the picture would have
been underexposed (i.e., "too dark" -- thanks, G). The areas
reached by the flash are exposed properly, but notice that the areas too far
away for the flash to reach are dark. Look at the histogram produced:
At a glance, we can tell that the picture has neither too dark nor too bright
pixels (i.e. it is "just right" -- Thanks again, G.) with most
of the pixels in the middle of the graph. There are a few pixels touching the
dark edge, just as we expected.
You might want to try dialing in a positive exposure compensation in this case
to see if the histogram improves. For the above picture, we'll dial in +1/3EV
and see if this shifts the histogram to the right slightly. Just be careful
not to overexpose it!
Breaking The Rules
Shutter Speed 1/160 sec. Aperture F8, +1EV, ISO 200
If you are aiming your camera at a landscape with dark shadows and very
bright sky, you may have both dark shadows and blown highlights (histogram
shows pixels touching both left and right sides). In this case, you may have
to decide which is more important to you. You are in control. In the picture
of the ferris wheel above, I even dialed in a +1EV exposure compensation to
try to bring out some details in the shadows, but this simply caused the bright
sky to become washed out. It would have been better to expose for the sky and
allow the girders to go black completely.
Shutter Speed 1/100 sec., F2.8, ISO Auto
For example, when we are taking a sunset, we usually do not really care about
preserving details in the shadows. In fact, we do want the foreground
to go completely dark. In this case, we expose for the bright area (the setting
sun) and let the shadows go dark for a dramatic sunset picture. In fact, we
may want to purposefully break the rules and take a couple more of that sunset
by dialing in negative exposure compensation values. This is because
sunsets usually look better (with deeper colours) with a slight underexposure.
(A note of caution about sunsets: never look at the sun directly, whether with
the naked eyes or through the lens of a camera. The image sensor of a digital
camera may also get damaged by the direct rays of the sun. When is it safe to
point the camera at the sun for a sunset picture? When your eyes can look at
the setting sun without requiring you to squint or block your eyes.)
If, on the other hand, you are more interested in preserving the details in
the shadows, you would expose for the shadows and let the highlights be blown.
Again, you maintain control, not abdicating it to the histogram.
Picture Blending
An advanced technique you can use is to take two pictures, one exposed for
the shadows and a second one exposed for the highlights (the use of a tripod
is essential here). Then you would edit, erase the under and overexposed portions
of each picture, and carefully merge the two pictures in an image editing software
(such as Photoshop) to obtain a picture that is properly exposed in both shadows and highlights!
Of course, this technique may be extended to taking as many pictures as you
feel necessary depending on how many difficult areas you have to deal with.
You certainly need to be pretty good at Photoshop.
Be careful, though, because this blending of many pictures sometimes result
in pictures that the brain innately recognizes as being "unnatural."
Another popular technique calls for taking a number of shots exposing for different parts of the picture and then running them through a HDR (High Dynamic Range) software such as Photomatix Pro [http://www.hdrsoft.com]. The result can be quite spectacular. Again, you have to be careful not to overdo this.
Professional photographers photoshop every picture, use software such as Photomatix, etc. to achieve the result they need. Believe me, their perfect photos do not come out direct from the camera.
Summary
The histogram is a handy graph that alerts you to the two extreme cases of
underexposure and overexposure in your pictures.
In severely underexposed pictures, you lose details in the shadows. In severely
overexposed pictures that result in blown highlights, you lose details in the
highlights.
If the image is too dark (histogram touches the left side), dial in a positive
exposure compensation value in +1/3EV steps, and see if the histogram shifts
to the right. Some pictures might require up to a +1EV exposure compensation
to preserve details in the shadows.
If the image is too bright (histogram touches the right side), dial in a negative
exposure compensation value in -1/3EV steps, and see if the histogram shifts
to the left. Some pictures might require up to a -1EV exposure compensation
to preserve details in the highlights.
Most digital cameras
allow you to dial in an exposure compensation between -2EV to +2EV, in 1/3EV
steps.
We have also seen that there is no good or bad histograms, per se. A histogram
simply tells us how our picture is exposed and we retain the control to decide
whether -- and how -- to adjust the exposure. We might decide to compensate
for the under or overexposure, or we might even force an underexposure if that
is the effect we are after.
Now, can someone please show G where the Three Bears are? Thanks a lot
for your help, G. Bye, bye now!
Editor's Note 1: An explanation of a complex
subject geared to beginners will of necessity makes some generalizations
and oversimplifications for the sake of clarity.
For example, a histogram that tends toward the dark needs not necessarily
indicate underexposure. In fact, if you take a shot of the moon at night
or other night scene with only a few sources of light, you will obtain
such a histogram, and it would be what you would expect and want. The
important thing in this case is to ensure that your main subject (the
moon, say) is correctly exposed. I'll have to cover this in more depth
in another tutorial.
To be techically correct, the histogram simply tells you how your image
is exposed. It strongly indicates that some part of your image may
be under- or over-exposed, but that determination is yours to decide.
You remain in control.
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