One of the most fundamental visualization tools in digital photography is the “histogram”. It appears on your camera display and it is used frequently in Photoshop. The histogram counts the number of picture elements (or pixels) contained in an image at each of the available brightness levels, with the darkest level on the left (displayed as black) to the brightest level on the right (displayed as white). Here is a histogram being generated from a very small digital image. In this case there are 36 pixels and 9 possible brightness levels:
The histogram reveals things that you might not have noticed when looking at the original image. Did you notice there were no 7s? In a real photograph there will be millions of pixels and hundreds or thousands of possible brightness levels, so the histogram will look much smoother than this small example.
The histogram can be used to judge the exposure and contrast of a photograph. If the histogram is piled up on the left hand side, the photograph is underexposed. Information in the shadows will be lost, or will be very noisy. If the histogram is piled up on the right hand side, the photograph is overexposed. Information in the highlights will be lost. The lost highlights cannot be recovered in Photoshop – a situation often referred to as “blown highlights”. It is important to make sure that important shadow and highlight information is captured in camera. A low contrast image will have a narrow histogram and a high contrast image a wide histogram spanning most of the available brightness levels.
If you can avoid losing important shadow and highlight information off the ends of the histogram, how the histogram should look depends on the subject and on what you intend to do with the image. An image of a dark subject (such as the night sky) is expected to peak more towards the left side of the histogram. Your camera’s autoexposure will try and put the peak in the middle, so you would normally compensate with some negative exposure compensation (or by using manual mode). An image of a bright subject (such as a snow scene) is expected to peak towards the right side. Here you would use some positive exposure compensation to get a good exposure.
Digital sensors are much noisier when working in the left hand half of the histogram, so if you are planning to use Photoshop it makes sense to try to push your image as far to the right as possible in camera without blowing the highlights or compressing the peak of the histogram against the right hand edge (which loses contrast). If you use Photoshop, it is better not to apply a negative exposure compensation to adjust dark scenes. Placing the peak exposure on the right hand side of the histogram will give you a less noisy exposure, and you can adjust the exposure level later in Photoshop. (The exception is when you genuinely need a shorter exposure, for example when avoiding camera shake is more important than avoiding noise.)
- Avoid losing important information by checking the histogram and making sure it isn’t piled up on the left or right. Some cameras have a “highlight clipping” display which can show you exactly what you are losing on the right. You can decide what is important – a blank background can be safely clipped.
- If you are not going to use Photoshop, adjust the exposure to place the peak of the histogram according to the scene – towards the left for a mainly dark scene, in the middle for a neutral scene or towards the right for a mainly light scene.
- If you are going to use Photoshop, place the peak of the histogram as far to the right as you can without clipping the highlights or compressing the peak against the right hand edge. (This technique is known as exposing to the right.)