The Digital Histogram

Having been a nature photographer for well over 25 years, I have seen many changes in the technical aspects of photography. Many of these changes have helped us to capture images that were difficult or nearly impossible. Because of auto focus I can now capture flight images that were almost impossible with a manual focus lens. Of course the digital revolution has also changed the way many of us now take images. For me one of the biggest benefits of a digital SLR camera is the histogram. The histogram is useful to help us in getting the correct exposure. Some people will say to use the image on the LCD screen, but the problem with that is the small screens makes it very difficult to see and also how the light hits the screen will affect your view. In order for us to use the histogram we have to understand how to read it. So let’s begin by looking at the histogram.

The Histogram

The histogram is divided into 4 sections.  Each section is “about” one stop of light. Going horizontal from left to right, commonly a binary count is used from 0 to 255.  These numbers represent the tone. O is black with no detail, 255 would be white with no detail, and the center of the graph would be neutral tone (18% gray).  So as an example when shooting a black bear, I would want the graph for the black bear to be in the column on the left somewhere in the middle of the column to the first line. If I was shooting a Snowy Egret I would want the graph for the egret to be in the column on the right somewhere in the middle or to the immediate line on the left. However if I was shooting a Great Gray Owl, which is very close to neutral tone, I would want my graph to be somewhere in the middle. The vertical movement of the histogram is telling you the number of pixels that are in that tone. The higher the vertical line the greater the number of pixels for that particular tone. For right now let’s not worry about what the binary numbers actually represent as that is too complicated to put in this article for now at least. What is also important about the histogram is any time the graph ramps up on the extreme left (0 count) or the extreme right (255 count), that part of the image is black or white, but with no detail. There is nothing a photographer can do even in Photoshop to bring back detail (short of patching). There are times on certain parts of the image that there is nothing a photographer can do about this.  But if the entire black bear or egret is in this area you have either under-exposed the bear or over-exposed the egret. Let’s take a look at a couple of images with their appropriate histogram.

Black Bear Histogram

	Click image for more detail.

This black bear lactating sow went up a tree during one of my workshops last summer. Her cubs were also safely in a tree right next to her. A male came through the sanctuary and she took refuge up in the tree. When you look at this histogram she is in the column on the left, which is where I want all of my black bear images to be. The tree and some of the leaves are in the center area for they are very close to neutral. If you look closely you will also see the graph ramping up to 255 on the far right. And I just told you that you want to avoid ramping on either end. But the Black bear was the most important part of this image so it was important to expose for her.  The over-exposed white is actually the birch trees but there is nothing a photographer can do - even digital makes it difficult to get detail in both white and black.

Bald Eagle Histogram

Click image for more detail.

Adult Bald Eagles always will challenge a photographer because they are full of contrast - a very dark body with a white head. Whenever I photograph Bald Eagles I do not want to over-expose the white head. When you look at this histogram you can see the head is right on the line and the body of the eagle is very close to 0, but not quite. For me this is a perfectly exposed adult bald eagle. I can see detail in the white head but still not lose detail in the darker feathers. In the center of the graph would be the rock with grass in the background.

There is really more to the histogram than what this short article is discussing, but this will give you the basics. Does this mean that I never miss an exposure because I use my histogram?  Obviously not as we all will miss exposures. But when I do look at the histogram I can almost always get within a 1/3 of a stop where I want to be if I am not right on. We all still have to understand the basics of exposure whether we shoot film or digital.  It is just that the histogram is another tool to help the photographer.

Good Shooting

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