There are several crucial means by which a photographer controls “the look” of his/her photos. Composition is the most obvious, but camera focus and depth of field run a close second. You may want everything sharp in the photo, or only what you want to draw the viewer’s eye to a specific subject… that pretty girl reading on the grass, for example.
You achieve this quality by controlling the depth of field, which is the front-to-back zone in the frame in which objects appear sharp. When little else but the main subject appears sharp, that’s known as “shallow” depth of field. When the image is sharp from front-to-back, that’s known as “deep” depth of field.
Three factors affect depth of field – lens aperture, distance from camera to subject and lens focal length. Aperture is most often used to control depth of field. To make as much of the scene as sharp as possible, set as small an aperture (f-stop) as the lighting will allow. To single out a subject, set as wide as aperture as possible; you’ll have to change the shutter speed to compensate in both instances.
The aperture ranges from f/1.4 (a super-fast lens) to f/22, with increments in between (f/2, f2.8, f/4, f/5.6, f/8, f/11 and f/16). Each f/number up or down represents one “stop” of light, such that when you move from f/4 to f/5.6 you’re letting ½ as much light, and if you move from f/16 to f/11 you letting in twice as much light. This number ratio is the same across all lenses, regardless of focal length… making it really easy to understand the concept across lenses. By understanding how the aperture works, you can utilize the hyperfocal distance for each lens for maximum effect. The hyperfocal distance is basically a point in which you focus and everything from that point to infinity will be sharp. When you want your landscapes to blow everyone’s mind, you have to master hyperfocal distance.
Another way to affect depth of field is to change the distance of the camera to the subject. To increase depth of field, move back; to reduce it, move closer. Think about how your eye works, the closer you are to an object the less you can see around you that’s in focus, but the further away you are, the more you can that is in focus. Of course changing your distance changes your composition, yet you could change lens or zoom the focal length to maintain the composition you want at the new distance. Now this negates any change in the depth field. Why is this?
At a given distance, at a given aperture, the longer the focal length of the lens, the shallower the depth of field is (the reverse is also true). The depth of field you get for a specific f-stop varies with distance, such that a small f-stop (like f/11) on a 35-70mm zoom set to 35mm gives a deep depth of field (from six to 20 feet with the lens focused at 9 feet), but you’ll have a relatively shallow depth of field with the lens zoomed out to 70mm (from 8 to 11 feet with the lens focused at 9 feet).
Just like with changing the distance from the camera to the subject, adjusting the focal to control depth of field is extremely difficult, too, because you won’t be able to maintain the composition that you wanted.
It is ideal to use the f-stop to control depth of field and shoot at your preferred distance with the focal length you want, because you can always alter the shutter speed (be sure to use a tripod and a cable release) to compensate for large f-stops, if you’re looking to obtain deep depth of field.
