How histograms can help you control key factors that affect exposure levels.
Correct exposures are the first step towards usable image files and the histogram displays provided in most digital cameras can help you to achieve them. Histograms show the brightness values in a scene in a graphical format, allowing you to see both how the exposure is positioned and the range of tones contained within the scene.
Each image histogram plots the number of pixels for each tonal value on the vertical axis against the range of tones on the horizontal axis. This axis is usually divided into 256 brightness values, which corresponds with the bit depth of 8-bits found in all JPEG files.
The left side of the horizontal axis represents the black and dark areas, the middle represents medium grey and the right hand side represents light and pure white areas. This means the histogram for a very dark (‘low key’) image will be biased towards the left side and centre of the graph, while the histogram for a very bright (‘high key’) image will be biased towards the right side of the graph.
The most critical area in any histogram is the tonal range, the portion of the graph that contains most of the pixel values. This range can vary greatly from one type of scene to another.
It is generally accepted that the majority of cameras can record a brightness range equivalent to roughly five f-stops, so if the difference between the darkest and brightest areas in the scene is five stops or less, you should record detail across the tonal range from the darkest shadow to the brightest highlight. But only if the exposure is correct.
When any of the auto exposure shooting modes (Auto, P, A, or S) is selected, most cameras will produce a histogram which peaks in the centre, gradually tapering off into the shadows and highlights. This happens regardless of the nature of the scene and it can sometimes produce exposures that aren’t quite correct.
When the scene contains a very wide brightness range, the tonal values can be pushed to the ends of the horizontal axis on the graph. Although most of the tones in the image shown in the bottom left corner of the opposite page are slightly to the left of the centre, there’s a large area of light tones hard up against the right hand end of the graph.
A typical brightness histogram, showing the tonal range for a scene with a fairly even distribution of dark and light areas.
High and Low Key Images
Images in which most of the tones are recorded in the shadows are called ‘low key’. They tend to appear dark overall, with only small areas of lighter tones. In contrast, images in which most of the tones are in the highlights are known as ‘high key’ images.
It’s always best to decide beforehand whether your subject matter qualifies as high or low key and decide whether exposure adjustment is needed to achieve your objective. Camera exposure meters measure the light reflected from the scene, rather than the light that illuminates it. Consequently, they can’t provide an accurate measurement of the absolute brightness of the subject; instead they measure the average brightness with a bias towards the midtones.
While this can produce acceptable results with ‘normal’ scenes, it won’t deliver the goods when you want either high key or low key pictures. You’ll need to take control and manually adjust the exposure, relative to what the camera would do automatically whenever you want the average brightness in your image to appear brighter or darker than the midtones.
Suppose you want to create a dark and moody picture? Exposing to bias the histogram to the left will achieve this objective. This means dialling in minus values of exposure compensation.
Want to go the other way and produce a light and bright picture? Adjust the exposure compensation towards the plus values.
You may need to swap metering methods because a multi-pattern metering mode will usually centre the tones on the horizontal axis of the graph, which averages them across the image brightness range. Choosing spot metering can help you to select an area in the scene that you’d like to record as a mid-tone. You can then adjust the exposure compensation to make it darker or lighter, depending on the outcome you’re seeking.
This image, which has a brightness range of slightly less than five f-stops, produces a histogram in which all tones fall inside the horizontal axis. Consequently, detail is recorded in both shadows and highlights. (Arrows indicate the regions on the graph that correspond to the brightness levels on the histogram.)
An image with a very wide brightness range, shown with the histogram that plots the tonal range. Note that the graph spreads across the entire horizontal axis, rising at each end and clipping both shadows and highlights.
In an image where dark tones predominate (‘low key’), the graph will be skewed to the left, as shown here.
In an image where light tones predominate (‘high key’), the graph will be skewed to the right.
The histogram is also useful for showing you whether tones are likely to be ‘clipped’ with the default exposure setting. While details may be recoverable in slightly clipped shadows, it’s usually associated with increased noise levels. At the other end of the scale, detail can never be recovered when highlights become so overexposed they ‘blow out’ to solid white.
Even with a small on-screen histogram, it’s usually possible to see when highlights or shadows are pushed to the edges of the graph. It will also show you the effect of any exposure compensation adjustments you make.
Some clipping can be acceptable in regions that should appear very bright, such as specular reflections on water or metal, white clouds, scenes in which the sun is included in the frame or when other bright sources of light are present. The extent of the clipping is up to the photographer’s discretion and should reflect the impression they wish to convey.
Histograms and Contrast
Contrast is defined as the difference in brightness between light and dark areas in a scene. This is another factor that can be displayed in a histogram. Since the height of the graph represents the number of pixels in a specific tone, the histogram can show you whether the scene is contrasty or relatively flat, and where the peaks in contrast lie.
Photos taken in strong daylight will have higher contrast, while those taken in mist or fog will have low contrast. Contrast can also vary for different regions within the same image due to both subject matter and lighting.
Using the histogram in conjunction with your camera’s exposure meter should allow you to obtain correctly exposed images very easily once you understand the role of the histogram. Almost all digital cameras, from the simplest point-and-shoot to the most sophisticated DSLR can display a histogram directly, either for live view shooting or superimposed upon the image just taken.
Set your camera to display a combined thumbnail and histogram for 5-10 seconds after every frame that’s recorded. Develop the habit of glancing at it to confirm whether image tones have been recorded the way you want. This lets you adjust exposure and re-shoot on the spot before the lighting changes.
The histogram for a contrasty scene shows jagged peaks that indicate tonal differences with deeper shadows and more pronounced highlights creating texture which ‘pops out’ at the viewer.
Smooth tonal transitions and low peaks are characteristic of low contrast scenes.
This is an excerpt from Photo Review Issue 55.