Law of Reciprocity in DSLR Cameras

F-numbers and shutter speeds work together to properly expose images. This is called the “Law of Reciprocity”.

Suppose you are photographing a gorgeous mountain landscape. The camera is set on aperture priority and indicates proper exposure is 1/125 second at f/8. Since the landscape has a near foreground and distant background, depth of field is needed to get everything sharp so set the aperture to f/22. You subtracted 3 stops of light by going from f/8 to f/11 to f/16 to f/22. To maintain proper exposure, the shutter speed must be adjusted to compensate for subtracting 3 stops of light with the aperture. Since the initial shutter speed is 1/125 second, slowing the shutter down 3 stops to 1/15 second maintains the best exposure. The original exposure of 1/125 at f/8 is exactly the same as 1/15 second at f/22 and the reciprocity law works. On aperture priority, the camera automatically makes this adjustment for you.

If you were using manual exposure, you have to make the adjustment yourself.