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In TTL viewing the lens is wide open, which gives us the minimum depth of field, but if the picture’s taken at a smaller aperture, other objects, both closer and more distant, come into focus. But how do you know what will be in focus? And how do you control it?

Traditionally, every SLR had a depth of field preview button, which manually stopped the lens down to the required aperture. This also made the viewfinder image darker, but it was still helpful in giving at least an indication of the effect on depth of field. These are relatively rare now, but digital photography does give us an instant review instead, and we can zoom in and examine this more closely if we’re really concerned about sharpness.

Depth of field is influenced by three main factors.

 The aperture Depth of field is smallest at the widest aperture, and increases as you stop down. There’s much more depth of field at f/16 than at f/4 (remember, they’re fractions). Most cameras have a Landscape mode, one of whose main aims is to set a small aperture. You can also use Aperture-priority for direct control. Just remember that aperture is only one of the factors in play.