The automatic exposure capability of modern cameras is a thing to be cherished. It usually produces excellent photos with nothing more than point-and-shoot. There are occasions, however, when it is best to override the automation to force a lower exposure. The built-in exposure automation assumes that the scene is reflecting 18% of the light falling upon it, and that nothing in the scene varies too much from the 18% average. There is trouble when one of those assumptions is wrong.

The most common exception is when the subject is darker than the assumed 18% reflective. Paroling the local farmer’s market last weekend, I set my pocket camera to underexpose by one and a quarter stops to take this picture of black plums:

What would have happened if I let the exposure proceed automatically? I added one stop back in Photoshop™ to see:

That’s what the dark plums look like when “properly exposed.” Had I let the camera run automatically, the image could have probably been saved in Photoshop by darkening it. Beyond saving the Photoshop work, there is an advantage in making the correction in the camera. The camera will either open the lens or lengthen the shutter speed to take the picture. With a pocket camera and a dark subject, the lens is probably already wide open. It’s a dark subject, so the shutter speed may go from 1/30th to 1/15th. That makes a blurred photo more likely.

Another case involves a bright subject against a dark background. In that case, the camera overexposes the subject to lighten the average expose up to its 18% standard. Here is an example of agapanthus flowers against a dark background. I set the camera to underexpose by one stop.

Again, I used Photoshop to see what an automatic exposure would have produced:

In this case, the normal exposure would have block the highlight detail in the white flowers, and one the detail is lost it cannot be recovered in Photoshop.

There are various ways to convince a camera to underexpose. Often there are settings for different types of subjects, and usually there is a way to lock the exposure either close or far from the subject before framing the shot you want. Advanced camera have selectable metering modes. I think the easiest way is usually to use the manual override button marked with +/- on the camera. That brings up a menu screen that allows adjustment, usual in the range of +2 to -2 stops from the setting determined automatically.

Automation is good, but occasionally it’s better to outsmart it.