Outdoor Photography. Chiz Dakin

of its importance. In all of photography, there’s no technical concept that’s more vital to grasp. And by tradition, landscape photography – photography of places – aims to maximise depth of field.

      This is not an arbitrary diktat. There’s sound reasoning behind it. As we’ve already suggested, when the shot has a clearly defined subject, like a portrait, it may not matter – it may even be a plus – if other elements in the frame are out of focus. But with general landscape views, everything is the subject. (Maybe the word ‘subject’ is almost as treacherous as that other word, ‘composition’.) In this kind of shot, a picture which is sharp throughout matches what we see, and looks more natural.

      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.

       The focal length of the lens The wider the angle (in other words, the shorter the focal length), the greater the depth of field. A 20mm lens has much greater depth of field than a 200mm.

       The distance to the point of focus Depth of field is greater when you’re focused on more distant objects. In real close-up work depth of field is minuscule.

      We might assume that an aperture of f/16 or even f/22 would be best. However, due to diffraction, overall sharpness tends to fall off at the smallest apertures. The best compromise between depth of field and sharpest results is often with the lens roughly half way open (eg at f/11 for a lens whose minimum aperture is f/22). At lower ISO ratings this means a slow shutter speed, which in turn means using a tripod or risking camera-shake. Today we have both image stabilisation and the option to turn up the ISO a few notches.

      The difference in depth of field is caused by changing the aperture from f/4 to f/8 (Chiz)

      Making the most of depth of field

      Remember that depth of field extends both behind and in front of the principal point of focus. If you focus on the most distant object in a scene, although depth of field is theoretically greatest, you’re only using half of its potential. The fact that more distant objects would be in focus – if there were any – is immaterial. In a typical landscape shot, the most distant part of the scene will be a skyline or horizon. Focusing on this is, as near as makes no difference, focusing on ‘infinity’. (Infinity has a somewhat specialised meaning in optics.) Depth of field theoretically extends beyond infinity, but this isn’t much use to anyone except Buzz Lightyear.

      You’ll often see advice in books and magazines to focus ‘a third of the way into the view’. The trouble is that no-one ever explains what they mean by this, probably because they can’t. I’ve never yet found an interpretation that really makes mathematical or optical sense. After all, what is a third of the way from here to infinity?

      With some very conventionally framed landscape images, focusing a third of the way up from the bottom of the frame can work, but this too breaks down as soon as you’re more imaginative about framing.