I was listing to a talk about improving your photography skills. The speaker asked the question,
“How many of you would like to put an image in a gallery with its histogram next to it?”
The question got a good laugh, but it was in response to the first things we are taught when starting to take more serious images is that a good images histogram has the classic half elliptical shape. The perfect histogram should arch from the shadows rising to the midtones and dipping to the highlights. This is a truth but a half truth in my opinion, to explain this half-truth I must start at the beginning.
What is a histogram?
A histogram is a graph that tells us how much of each tone of light there is in an image and for each color. A histogram for an RGB image will have a histogram for each color plus one with all the combined colors representing light. The graph ranges from blacks through to shadows on the left, with the middle representing the midtones of the image and the right side being the highlights through to highlights on the right. The height of each tone shows the amount of tonal presence in the image. Using a histogram you can say that the classic half elliptical shape is a perfect exposure as the details mainly being in the midtones with a small amount of highlight and shadow adding shape and form. If the histogram shows that most of the information to the left of the graph then the image is underexposed and to the right overexposed. Like all things this graph is a guide to tell you want is a good exposure but does not mean to say that it is the right exposure.
It is always a good idea to have your playback screen showing you the histogram so you can review the shot on its light quality. Usually when I shoot with my Compact (G10) I use the histogram to help me guide my exposure in live view, whereas with my DSLR I have it showing on playback.
Of course you need a guide but also you need to use your brain. The camera can only tell you want is there but you need to assess this information to help create your shot.
With this shot of a bee on a thistle the exposure looks good. The bee and the flower have good contrast and sit in the midtone range with details being formed in the shadow and highlights. This goes to explain the main bulk of the tones sat in the middle of the histogram. Yet you can see in the picture there is a slight vignette effect with the fall out of the focus creating the small highlights to the right of the histogram. The bee has also helped create the shadow tones on the left, because of the black part of its body. If I exposed the shot for longer, I would lose some of the contrast detail in the flower that makes the bee and the flower pop and stand out from the background. In under exposing the shot it would cause the same to happen but shifting everything the other way. For this image the idea of the good exposure is correct.
Fugly fish on the other hand is a very different. Looking at the image the fish is the main subject and it is his exposure which is important. If you were to look at the histogram you would expect the image to be under exposed with a spike of highlights. Yet looking at the picture it does not look underexposed.
Where does the under exposed expectation come from?
Well the huge area of dead black space. This accounts for most of the shadows in the histogram graph. The spike in the highlights is from the water pump churning the water in the top left of the image. With all this though the most important thing concerning the exposure of this image is the exposure of the fish.
In zooming in on the fish and cropping I can see how the histogram changes. Of course the black and the highlight spike are still causing there problems but the rest is sitting quite nicely spread across the graph. The highlight/midtone detail is found in the fins with the yellow as well as the front of the fish’s face. There is some shadow/midtone detail in the underbelly of the shot; these shadows could be raised to create tonal fidelity in the image.
Overall the histogram says the image is underexposed yet in fact for this image it is ok.
With an over exposed image the histogram should be shunted to the right this because the main detail in the image should be clipped in the highlights with all detail at this end of the spectrum. The image shot over Wisła River in Torun has an interesting histogram. The histogram is heavy weighted to the right with a little bump in the shadow midtone area. This type of histogram would generally mean that the image is overexposed.
In this case the image looks the opposite. The image is not evenly balanced but trees have a light darkness to them that are reflected black in the river itself, along with the darkness of the river, this makes up the bump in the lower midtones. The highlight shift at the end is the result of the blown out sky. The detail of the sky has been clawed back slightly due to the vignette.
Histograms are an important part of digital imaging and they have their place. They tell us what information there is and isn’t in an image. In adding contrast to an image the histogram will create gaps as tones are moved to brighter or darker areas to create the contrast therefore showing a loss of information. The same as when you add saturation, generally a histogram will drop in height flatten the color tone to similar values.
A histogram is not the be all and end all when it comes to the perfect exposure, though as with most things in photography it is a useful guide. When shooting and checking your images on your LCD screen it is worth having the histogram on to check the histogram is right for the scene.
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