Part 16 - Exposure Compensation

We've seen that the DSLR camera's histogram accurately displays the density distribution of each image immediately after we take a picture. This function alone makes digital photography technically easier than traditional film camera -- especially for those new to photography.

Histogram display of a Nikon D200

We've also learned that keeping the range of densities within the borders of the histogram produces the best image exposure. The most effective way of controlling this positioning of densities when judging the histogram is by using your camera's exposure compensation control.

Canon D30 LCD Display

The exposure compensation scale appears in your viewfinder or the LCD control display, or BOTH. The display above is from a Canon DSLR. The exposure compensation scale is located on the lower right corner of this display. In the position shown, with the indicator set at zero (highlighted in red), NO exposure compensation is being applied.

Notice the scale range is from minus 2 to plus 2 with half way points between each number on the scale. In photographic terms, this range translates to two full f/stops of adjustment on each side of zero (no correction) in one-half f/stop increments.

Let's look at exposure compensation applied to a real picture taking situation. The picture above was taken in aperture priority mode using the exposure suggested by the camera's meter. The result is a picture that's too light. This is verified by the camera's histogram where the majority of the densities are "pushed" toward the extreme right side of the graph. In this case, the picture needs to be made darker by intentionally overriding the camera's suggestion and UNDEREXPOSING the image.

Setting this camera's exposure compensation dial to the minus side of the scale will provide this intentional exposure adjustment. In the picture above, the exposure compensation has been set to Minus 1. This selection will override the meter setting by doubling the shutter speed (for example from 1/125th of a second to 1/250th of a second) without changing the aperture (f/stop) setting. Remember, in this example the camera is set to aperture priority mode.

If the first exposure was too dark, the process is reversed. The histogram display for the dark image below indicates that most of the densities in the image are pushed toward the left side of the graph.

Now the objective is to "lighten" the picture by allowing MORE light to enter the camera (or intentionally overexposing the image).

In the display above, the exposure compensation scale has been set to Plus 1. The result will be the slowing down of the shutter speed by half (for example 1/125th of a second to 1/60th of a second) without changing the aperture setting. This allows twice as much light to enter the camera to create the final and lighter exposure.

Mechanically, the exposure compensation on this Canon DSLR is made once the shutter button is pressed half way down and the master dial on the back of the camera is turned clockwise or counter-clockwise. Most cameras work in a similar fashion. Read your manual for the operation of this feature on your camera.

The object of making either of these exposure adjustments is to correct the distribution of the densities displayed on the histogram to be within both the left and right-hand sides of the graph.

The normalized histogram in the picture above has been achieved by using the exposure compensation control. With all densities well within the borders of the graph, this image is printable with good detail in both the shadow (left hand side of the graph) and highlight (right side of the graph) areas of the picture.

Note: In the examples above, adjustments of Plus 1 and Minus 1 were used. The exact amount of correction required will vary depending on the situation. Experiment with adjustments until the histogram density distribution "moves" to the correct position. You will eventually get a "feel" for where you should start with your adjustment.

The huge upside to taking your time to determine the proper exposure and applying exposure compensation as required is that you will leave the shooting situation KNOWING you have a good printable image -- something that could not be said with 100% certainty in traditional film-based photography.

Spend the time, and reap the rewards.

If you have any questions or comments, let me know.