Understanding the Digital Histogram
By Blair
Howard
If you shoot
digital, you have a distinct advantage over the film photographer. One of the
most helpful features of the digital camera is the histogram, if you know how to
use it.
The Digital Histogram is not nearly as scary and difficult as it sounds.
It is really nothing more than a graph that shows how the image has been
exposed: its brightness.
Learn how to read a histogram and you will never need to worry about
washed out highlights and blocked shadows again. It’s not nearly as difficult as
it seems.
Every medium to high-end digital camera incorporates this feature. Many
point-and-shoot cameras have it as well. Photoshop and Photoshop Elements can
also display it. If you don’t know how to display it on your camera, stop
reading this for a moment and get out the manual and learn.
Even though you can’t view it on your 35mm camera, the histogram applies
to negative film and transparency film as well. To view a histogram of your
transparencies or negatives, you’ll need to scan them and view the results it in
your “Digital Darkroom” (Photoshop).
The histogram is simply a graph showing how the light was recorded from
pure black to pure white and everything in between.
There’s an awful lot of information available in the histogram, but for
our purposes, we’re going to use it for one thing only, to find out if we’ve
blown out our highlights or blocked out our shadows.
Bearing in mind what we (should) already know about highlights and
shadows, you know that we don’t necessarily want to completely avoid either of
those things. We do, however, want to know if we have done them. If we’ve
totally blown either one, the resulting image will be useless.
There will be times, however, when we may want push the highlights toward
the upper level in order to bring out some detail in the shadow areas, and
sometimes we may want to underexpose the shadow areas somewhat in order to tone
down a washed out highlight.
Reading the histogram will tell us if we’ve achieved the results we were
looking for so we can move on to the next shot.
How to Read
the Histogram
In the simplest of terms:
The left side of the chart indicates pure black; the right side, pure
white; the center of the chart is the mid-range. So:
If you look at the histogram you’ll see the background is white. The
indicator – the peaks and valleys – is black. The position of the bulk of the
indicator and its highest peaks should, in most cases, be close to the center of
the chart. If this is the case, you do, for all intents and purposes, have a
correctly exposed photograph.
If most of the peaks and valleys are all the way to the left and
disappearing off the chart, you know you have underexposed the image and the
shadows will be blocked out, but you will have detail in the highlights.
If most of the peaks and valleys are all the way to the right and
disappearing off the chart, you know you have overexposed the image and the
highlights will be washed out, but you will have detail in the shadows.
Below is a list of some of the other things the histogram can do for us,
most of them in Photoshop or Photoshop Elements. Obviously, we can’t discuss
them here, but you do need to understand just how important the histogram is and
how it can help you to improve your photography – digital or film.
None of these things can be done when you’re out in the field shooting.
You can, however, set your camera to its review function so that it will display
the histogram and leave it there. Now when you take a shot you can check the
histogram and know immediately if you’ve overexposed or underexposed the image.
You’ll also be able to see if you’ve washed out the highlights or blocked the
shadows.
Suppose you decided you wanted to overexpose the highlights a little and
capture shadow detail? Perhaps you were more concerned with retaining the
highlights and letting some shadow areas go dark. Whatever, in less than a
second you’ll know if you got the results you wanted.
If you missed the shot, you can shoot it again. If you have what you’re
looking for, you can move on and shoot the next image. If you’re shooting the
same subject, but maybe from a different perspective of viewpoint, and if the
lighting hasn’t changed, you don’t need to look at the histogram again; you
already have the information you need and can continue shooting with confidence.
Often, especially in high-contrast situations, I’ll check the histogram
after every shot. Then again, when conditions are not so extreme, I’ll check it
maybe only once in every 50, or so, shots. I do, however, always check it on
the first shot of a new location. That, my friends, is crucial. More times
than I can remember, that simple step in my routine has saved me from blowing an
assignment.
The histogram is an amazingly useful tool. The more you use it, the more
you’ll like it. Best of all, your photography will improve significantly.
Quick Tip:
Though not as useful as reading and understanding the histogram - but
extremely when you are in a hurry - there is another tool available in most
digital cameras. You can set your camera to show “blinking highlights”. Do this,
and when you look at the review screen, you see the image just as always, but if
there are areas where you’ve overexposed and blown out the highlights they will
blink either red or white, depending upon the brand of camera.
Blair Howard
is a professional photojournalist
www.blairhoward.com and chief creative officer and instructor of digital
photography at www.AIOPonline.org.
This article is copyright
© Blair Howard 2006. All rights reserved. You can
reprint this article for free but you must retain the internal links and the
credit line at the bottom of the article intact.
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