April 7 2007
Digital Histograms help photographers adjust exposure settings on digital cameras in the field.
One of the benefits of some digital cameras is they have a form of immediate feedback on exposure. Digital cameras display an exposure histogram, as well as the review image on their LCD screens.
A feature of sophisticated larger format film cameras is an interchangeable film backs that allows a shot to check exposure with instant Polaroid film. The photographer can then swap back to a normal film back for the final shot. This is where the histogram comes in, a simple graph that displays the brightness levels in the scene, from the darkest to the brightest.
How Histograms Work:
Digital images are numbers, and a scale from 0 to 255 represents the exposure levels. The histogram displays the 256 brightness levels along the horizontal, or bottom axis with the blackest black on the left, and the brightest white on the right. The height of each column on the vertical axis indicates the number of pixels at each brightness level.
If most of the pixels are near the left side of the histogram and none near the right then the image is under exposed. The reverse is the case for over exposure, with pixels crowding the right of the display. One term used to describe aspects of over exposure is “blown highlights” where there are areas of the image that are whiter than the level at 255 and they get assigned to the 255 pile as there are no higher options.
Applying it to your Photographs:
On board metering systems are very good for simple lighting situations where there is an even level of brightness. The histogram reflects this with an even spread of pixels over the whole range. The LCD screen is not a good guide to exposure, especially in bright conditions.
Exposure meters in modern cameras sample light from areas in the scene and, after applying their own emphasis, arrive at the final exposure setting. This sampling may be centre weighted, where an area at the centre of the viewfinder is the most important area for determining exposure. Alternatively, matrix metering, where a number of spots spread over the whole view are used, and the exposure information combined to give an overall metering solution for the image.
The scene might not look difficult, but when the spread of light does not match the built-in exposure rules programmed into the camera’s metering system there can be problems. This may show on the histogram as slight crowding at one end. Taking a second shot with a slight adjustment using exposure compensation may just move the exposure a little towards an even spread across the histogram.
For example, there is the issue of blown highlights, where small areas of lighter colour are over exposed, even though the exposure of the scene overall is acceptable. On the histogram, this looks like a spike on the right side of the display. This is where the photographer makes the decisions on the proper exposure level for the areas considered important. This underlines the advantage of digital cameras that allow the photographer to make on-the-spot reviews of test shots before shooting the final shot.