Controlling Depth of Field (DOF)

Every photographic lens that has glass in it has something called Depth of Field (DOF). Simply explained, DOF is that additional distance (both in front of and behind the object the lens is focused on) which is also in focus. In general, the distance that's in focus behind the focus object will be much greater than the distance that is in focus in front of the focus object. In some cases everything behind the focus object, all the way out to infinity, will also be in focus while in other cases the DOF in front of the focus object may only extend a few feet.

Some lenses have a very shallow or narrow DOF, some have much larger DOF. The amount of DOF will vary from lens to lens, depending on focal length and manufacturer. For example, all Canon 50mm f 1.4 lenses will have the same depth of field but the depth of field between a Canon 50mm f1.4 and Nikon 50mm lens may be different. And the DOF will be different between any 20mm lens and any 300mm lens. Knowing about DOF and how it will affect your pictures will allow you to determine and choose whether your background will be sharply in focus or blurred and indistinct, also what part of the foreground will be in focus or blurred. It’s not as simple as learning if your lens has a small or large depth of field and then just remembering that, there are two primary things can change the apparent depth of field on any lens. These things (F-stop and the distance at which the lens is focused) can be used to determine what the DOF in your picture will be.

The Effect of f-stop

The first thing that will control the depth of field is the f-stop, or how open or closed the lens aperture will be when the shutter fires. The smaller the f-stop number, the more open the aperture will be. In the same way, the larger the f-stop number, the more closed the aperture will be. Larger apertures (lower f-stop numbers) lessen the depth of field of a lens; smaller apertures (higher f-stop numbers) increase the depth of field of a lens. So the first step in controlling the depth of field is determining what f-stop you will use based on whether you want a sharp background or a blurred background.

At this point, you must take the amount of light available into account when doing this. If you close the f-stop too much --select too high a number-- for the light available, your picture may be underexposed if the camera cannot slow the shutter speed sufficiently. Selecting a large f-stop will require the shutter speed to decrease. This can lead to motion blurring of the picture caused by hand-shake and other factors. For situations where you will be shooting at a speed slower than 1 over the lens rating, always use a tripod. (i.e. you should never try to hand hold a 50mm lens for example at speeds slower than 1/50 of a second, in the same way you would need to shoot a 100mm lens at 1/100 of a second of faster).

The Effect of Distance from the Subject

The other thing that will modify DOF is the distance between the focus object and the camera. In general the closer the camera is to the focus object the narrower the DOF at any given f-stop. Most lens manufacturers put a DOF scale on their lenses. Normally this is a small window on the barrel of the lens. There will be some lines and numbers etched into the barrel above or below this window. The numbers displayed are f-stop numbers. Through the window you will see figures (normally in feet and meters) indicating the distance to the focus object. The lines on the barrel indicate the distances (at the indicated f-stop) which will be in focus in front of and behind the focus object. This DOF scale is often hard to read and may not contain all f-stops. DOF charts are often available from the lens manufacturer or third parties, but the best knowledge comes from experiment and experience.

The pictures in the table below illustrate what we've been talking about and the labels should be self-explanatory. Take the top line of pictures for example. All were shot at f1.4 but each was shot with a different focus point. Notice how the writing in the black square at the right of the picture is sharp in the picture with the near focus point while it is blurred in the picture with the far focus point. This is a clear case of the near depth of field being to short or narrow to display that writing as in focus. In that same line of pictures, notice that the furthest away parts of the picture with the near focus point are not as clear and sharp as they are in the photo with the far focus object. In that first picture, the near focus object one, the depth of field is too narrow to display that portion of the picture as sharply focused.

Near Focus Object Mid-Range Focus Object Far Focus Object
* indicates approximate focus point


As I said earlier, the best knowledge comes from experiment and experience. However, in general, if you want a blurred background, open up the lens aperture (small f-stop numbers) and get close to the focus object. To make the background sharp do just the opposite.