When you pick a point of focus in a scene, DOF isn’t evenly distributed in front of and behind the subject. Generally, one-third of the available DOF will fall before the point of focus, and two-thirds of the DOF will fall after the point of focus. This guideline may not be a hard and fast rule (because the ratio changes with distance), but it makes for a great way to apply DOF selectively within a scene.
If there are particular items nearer or farther from the point of focus that you hope to place within the zone of sharpness—a landscape, for example, with items in the foreground and in the distance—it may be best to move the focus point. That’s right: Sometimes it’s best not to focus on the most important element in the scene. It all depends on those supporting details and where they sit in relation to the subject.
For example, if you’re photographing an approaching sleigh pulled by nine reindeer, you might choose to focus on the bright-red nose of the lead reindeer. The DOF then might extend all the way to the third or fourth reindeer (depending on the lens focal length, aperture and distance). But because the DOF extends into the foreground, too, you’re wasting part of the usable DOF on the area in front of the lead reindeer, where there’s literally nothing in the scene!
If you adjust your point of focus from the lead reindeer to the second or third reindeer, for example, now you can take full advantage of the available DOF and get the whole team into sharp focus, from Rudolph up front all the way to Blitzen at the back. By picking a point of focus approximately one-third of the way into the zone you would like to keep in focus, you use all the available DOF to its fullest advantage.
Check the math; get your hands on an old manual-focus 35mm film SLR and lens. The markings on the top of the lens allow you to measure how much of the scene will fall within the sharp DOF at a given distance and aperture. Maybe it’s because DSLRs with LCDs allow you to check your DOF right on the back of the camera, or maybe it’s because the DOF preview function is so simple to use, but most modern DSLR lenses no longer include this manual DOF scale.
Focusing one-third of the way into a scene doesn’t guarantee that the entire frame will be sharp. You still have to utilize aperture and focal length appropriately to maximize DOF. But for a given zone of sharpness, positioning it properly does mean that you won’t waste any available sharpness that the aperture and focal length provide.