If I have an overexposed photo with a severe amount of white light, my histogram will display a graph with a spike touching the rightmost portion of the histogram. The opposite would be true for an underexposed photo.
These histograms will also often be missing the information for blacks or whites, depending on if your photo is over- or underexposed.
So, for instance, in an underexposed photo, the photo might contain no information for whites or highlights. As a result, the graph would be completely absent from the left side of the histogram. The reverse is possible for overexposed photos.
An underexposed photo with no whites or highlights. Notice how the graph touches the left edge of the histogram and lacks any information for the whites or highlights, leaving that right side of the graph empty.
An overexposed photo. Here, the graph touches the right edge of the histogram. Because of a lack of blacks or shadows, the graph does not extend to the left of the histogram.
In most cases, you photos will not be as radically over- or underexposed as the photos above. Nevertheless, if you notice a spike touching the edges of your histogram, you should make adjustments if you would like to achieve a more neutral exposure without missing details.
Ideally, you should take several different exposures of the scene or subject you’re trying to capture, checking your histogram after your first shot to validate your exposure settings. This will allow you to make adjustments with plenty of time to spare in the case of an incorrect exposure.
Should You Always Use Histograms to Correct Your Exposure?
Well, yes and no.
Just because a photo produces a histogram inclined towards a certain tone, that does not necessarily mean your photo has an incorrect exposure. The readings that matter to your photo’s details are whether a peak of your graph is completely touching the right or left end of your histogram. It is absolutely possible to have a correctly exposed photo that leans towards shadows or highlights.
Aside from all the technical mumbo jumbo, you can also stylistically choose to shoot a photo that breaks all the rules and disregards a histogram’s suggestions. Technicalities like proper exposures and their histograms should, within reason, be used at your discretion as an artist.
Take into context what kind of exposure best suits your subject. A landscape, for example, would probably benefit best from an exposure setting that produces a neutral histogram. A portrait, however, might be dramatized by a darker, or more highly contrasted exposure.
Your choices rely on your awareness of what mistakes can be made, both on the part of your camera’s meter and the histogram, and what aesthetic choices you have in mind.
This doesn’t, however, make your histogram obsolete. Histograms can go beyond just ensuring your photo is correctly exposed. In fact, it benefits you to know what else your histogram can tell you about your photo and when your photo might actually be the exception to the rule.
High Contrast Histogram
High contrast photos make use of plenty of strong black and white tones and fewer midtones. They risk losing details for what may be a more impactful photo.
Histograms for a high contrast photo will have strong, peaked readings for dark and light tones, and lower readings for the middle grays. The shape can almost be comparable to a ‘U’.
This kind of histogram reading doesn’t mean your photo is bad! High contrast is often used in photography to add a dramatic mood to photos, and is very popular in the genre of black and white photography.
While most editing programs will allow you to artificially bump up the contrast of your photo, it’s completely possible to shoot with high contrast in mind. Scenes with harsh shadows and lights, or sharply differing color contrasts can all produce high contrast photos straight from your camera.
A high contrast histogram generated by the harsh amounts of blacks, shadows, whites, and highlights in this photo – no editing required.
If that happens to be your goal when shooting, keep in mind that, while the appearance of your histogram might correctly suggest the loss of detail, you’re actually looking to generate a histogram with more blacks and whites than midtones.
Low Key Histogram
Low key photography is very similar to high contrast photography in that low key photos often add a more dramatic mood to your pictures. Low key scenes are shot with a generous amount of darks and blacks and a reduction to highlights and whites. This doesn’t mean that whites can’t or shouldn’t appear in your photo, simply that dark tones are far more common. The graph for these photos will be inclined to the left of your histogram, with a heavier emphasis on dark tones and midtones.
Low key photos can also be artificially created through editing, or produced straight from your DSLR with the use of either natural or studio lighting.
Since the histogram won’t match a proper exposure’s one, it’s on your judgement to decide whether you’ve taken an adequate photo or not.
Low key photos may also produce a graph that touches the left of your histogram.
In the example above, the portrait of the dog has a completely flat black background, with no perceptible details. As a result, the portrait registers as having lost detail and generates a histogram with a graph that touches the leftmost point of the histogram.
As with the example of high contrast histograms, this doesn’t exactly mean that this particular photo is underexposed, just that there are black, plain tones found within the photo. Because low key photos involve a heavy amount of dark tones, it is acceptable for your low key graph to touch the left of your histogram.
High Key Histogram
Just as you’d expect, a high key exposure is the opposite of a low key exposure, reducing the shadows and blacks of a scene, and instead focusing on highlights and whites. Like low key photos, high key scenes can continue to utilize smaller amounts of dark tones. The graph of a high key photo will be shifted to the right of your histogram.
High key photos can also contain informationless white tones that are a perfectly normal, acceptable part of the photo.
Also like low key photos, high key photos can be used to convey moods, most popularly, happiness. Even in the photo above, the lighter tones contribute to an airy and relaxed vibe. It’s a good example of how context affects your photo and your stylistic choices. Context is a key aspect of how you choose to shoot your photo, interpret your histogram, and make further edits.
You can create high key photos through natural lighting, studio lighting, and/or an editing program. Again, it is up to you to evaluate your subject, determine if it is in fact a high key scene, and make adjustments to produce an accurate photo.