In addition to regulating your exposure, the aperture also controls the depth of field, the range of good focus from near to far away. Wide apertures, like f2, allow lots of light to reach the film, but they give you very little depth of field. If you want both nearby objects and distant ones to be in sharp focus, you’ll need to “stop down”, that is use a small aperture, a setting like f11 or f16.

Since we have two separate ways of controlling exposure, we can choose from a number of equivalent exposures, different combinations of aperture and shutter speed settings which allow the same amount of light to reach the film.

For example, let’s say we are walking on a beach. There’s an interesting piece of driftwood at our feet, and neat cliffs with swirling fog in the distance. With a 50mm lens, we have a nice composition. We determine that 1/250th at f5.6 is a good exposure for our possible image, but we want more depth of field. We decide to stop down to f16. We’re closing the aperture three stops, so we’ll need to use a shutter speed that is three stops slower; we go from1/250th to 1/30th. Less light gets through the smaller aperture setting, so we need a longer shutter speed to compensate.

Most cameras calculate the equivalent exposures for you, so you don’t need to worry about the mechanics of the calculation, however; it’s important that you understand the concept of equivalent exposures and the trade-offs involved.

In our example, we could handhold the shot with the 1/250th, f5.6 exposure and get a tolerably sharp image. But, we’d have to choose whether the driftwood or the cliffs would be the sharp part, because we couldn’t get sufficient depth of field at f5.6 to get them both in clear focus. At 1/30th, f16, however; we can get the whole frame sharp, but we’d need to use a tripod.