The Histogram Made Simple
Some of the most common questions we get at the workshops we teach are about histograms. The histogram is an extraordinarily useful tool in digital photography. We encounter it in our image editing software as well as on the LCD screen of our camera when we review our images.
On Nikon cameras, while reviewing an image, press the multi-selector up or down to get to the window that shows your image with the accompanying histogram.
We utilize the histogram while reviewing an image just shot, to assist us in determining whether the exposure is correct. The histogram by itself does not tell us whether the exposure is good or bad, rather it requires an interpretation based on the tonal scale of the scene we are photographing. It is this interpretation that confuses people.
The simplest explanation for a histogram is that it is a bar chart that reflects a tonal scale from black to white, left to right, on the horizontal axis and the amount of data, none to a lot, bottom to top, on the vertical axis, that resides in each part of the tonal scale of your image.
This is the histogram for the 3-step card above. It shows that the image has all its data in three areas: black (far left), dark mid-tone (center) and light mid-tone (right). The little blip at far right in the histogram correlates to the two thin white strip that separates the three tone patches.
The shape of the histogram is a combination of the range of tones in the scene and the exposure of your image. Because of this, there is no perfect histogram that would apply to every image. Instead, what we are looking for in the histogram to confirm that we have made a good exposure, is that every part of the tonal scale that is present in the scene you are shooting, is present in the histogram as well.
In a scene with a full range of tones, a histogram confirming a proper exposure would reflect that full range of tones by showing data going all the way from left to right, black to white. Many photographers, myself included, feel that a good histogram should not go all the way to the right, but rather stops just shy of the far right end to allow more flexibility in image editing and to help avoid blown-out highlights. Don’t be too concerned with the vertical axis (how high the peaks in the histogram are). The vertical axis simply shows how much data is present in each part of the tonal scale. What we are more concerned with is that there is data present in each part of the tonal scale that is present in our scene, not how much of each part of the tonal scale is present.
The key to interpreting a histogram is understanding the nuances in tonal scale in the scenes you photograph. Most scenes contain a full range of tones, which make it fairly easy to interpret the histogram, but some scenes may be composed predominantly of dark tones or predominantly of light tones. Good histograms for these different types of scenes will look completely different.
The scene photographed contains a full range of tones going from black to white (left to right in the histogram). The histogram reflects a good exposure because the tones in the scene are represented in the histogram. If you shot this photo and looked at a histogram that showed the data not going almost all the way to the right, that histogram would be showing you that you were underexposing the scene.
The scene is composed mostly of dark tones. There are no bright mid-tones or bright whites. The histogram reflects that accurately. It shows that most of the tones in the scene are dark and many will contain no detail at all.
The scene photographed has a full range of tones but a predominance of light tones. The histogram reflects that accurately.
Since it is often difficult, especially in bright sunlight, to objectively assess the exposure of our images by looking at them on the LCD screen on the back of our camera, the histogram is a far more useful tool for us because once you know how to interpret it, the histogram can be used as a completely objective tool to evaluate exposure.
After you’ve taken a photograph, bring it up on your LCD screen and navigate to the histogram. Interpret the histogram, based on the tonal range of the scene photographed to be sure that data is present in the histogram in each part of the tonal range that you see in the scene.
In a scene with a full range of tones, you should see data present in your histogram from close to the left side to close to the right side. If the data ends far from the right side, your image is underexposed. Shoot another photo increasing you exposure. If using one of the auto exposure modes, use the EV+- button to override the camera’s exposure meter. EV+1 to increase the exposure in your next shot. Check the histogram on your new image and keep tweaking EV until you get it right. If all the data seems ganged up on the right side of the histogram and doesn’t extend almost all the way to the left edge, or even if it does extend all the way to the left edge and it seems like you have a big spike on the right of the histogram, you may have an overexposed image. Try shooting another frame with the EV set to EV-1. Repeat til you get a good exposure, tweaking the EV as necessary. Don’t forget to reset the EV to EV0 when done!
Our initial exposure here was F/5.6 at 1/60th. The scene has a full range of tones but weighted heavily towards the brighter white tones. The histogram shows the tonal data shifted too far to the right. We compensated in our next exposure by dropping the exposure three stops to F/5.6 at 1/500th….
Oops! Looks like we overdid it. Because of all the white tones in the scene, the histogram should extend all the way to the right, but in this image, the histogram drops off in the mid-tones. We need to increase our exposure in our next shot so we change our exposure once again, this time to F5.6 at 1/125th….
Bingo! Our exposure and histogram are now accurate. Accurate is not always the same as good, so I may have tweaked the exposure just once more by reducing it 1/3rd stop to leave just a bit of room at the right of the histogram but we’re close enough.
I hope that helps explain the histogram. The only way to really understand the histogram is to get out and learn it by doing. So get out there and get shooting!
Photos and story ©2010 Michael A. Schwarz. No reproduction without permission.
Michael A. Schwarz is a commercial and editorial photographer, digital photography consultant and photo workshop leader based in Atlanta, Georgia.
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