Using Histograms as an Exposure

Correct exposure is the key to great photos. Overexposure should be avoided at all costs, as washed-out highlight detail cannot be recovered at any stage during the workflow.

In order to help you judge exposure, digital cameras allow you to check your exposure visually on the monitor immediately after shooting. Nearly all digital cameras - at least all DSLR cameras - have a histogram display that shows the distribution of tonal values within the image from 0 (black, on the left) to 255 (white, on the right).

On the next page are four sample histograms produced using Photoshop that show three typical curves.

The histogram in figure 1-13 shows an overexposed image with obviously clipped highlights at the right end of the graph. This image will lack highlight detail. Images that display a histogram like this are generally only good for deletion. Some photographers recommend using the Photoshop Burn tool to darken images like this, but the tool only simulates detail that wasn't actually present in the original image.

The histogram in figure 1-14 shows only a small overexposed peak on the far right. The actual subject will dictate whether this amount of highlight clipping is acceptable or not. If the clipped highlight is pure white or represents a less important image detail, the photo might be OK. Otherwise, it may be safer to ditch it and re-shoot.

The histogram in figure 1-15 is of a balanced exposure. The missing tonal values at the white end of the scale can be reconstructed using Photoshop. You should always aim to produce images with histograms that look similar to this one - i.e., with an even, unclipped grayscale "mountain" as far towards the right-hand end of the graph as possible. This approach is often described as "exposure to the right".

The next section will clarify why it is better to underexpose than to risk losing highlight detail, especially as it is often difficult to view and judge a histogram correctly in bright light or on the road.

> Remember that the monitor image and the image represented by the camera histogram are in JPEG quality and have already been subjected to a RAW-to-JPEG conversion by the camera's firmware. If you are shooting in RAW mode, your computer-generated histograms and the resulting images (e.g., in a RAW editor) will often look different from the ones the camera displays while shooting.

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