Black and White Photography

The term depth of field refers to the depth of the zone that is visibly sharp in

the picture, from the closest to the farthest parts of the scene. Suppose you

focus your lens on a tree 10 feet away. Even though you focus precisely on the

tree, an area in front of and an area in back of the tree also will usually be

sharp. The degree of that sharpness, from front to back, is the depth of field.

The depth of field of a picture may vary widely and is controlled by these

factors: lens aperture, distance to subject, and lens focal length.

Lens aperture. The smaller the lens aperture you use, the greater the depth of

field. Thus if you set your lens at f/16, you will produce an image with far

greater depth of field than if you set the lens at f/2, other factors being equal.

Lens aperture is probably the most understood factor in controlling depth of

field, but the next two factors are just as important.

Distance to subject. The greater the focusing distance (from camera to subject),

the greater the depth of field, assuming the lens aperture and focal length stay

the same. If you use a 50mm lens and focus on a subject 20 feet away with the

lens aperture set at f/8, you will get much more depth of field than if you focus

with the same lens at f/8 on a subject five feet away.

Lens focal length. The shorter the focal length of the lens, the greater the depth

of field. If you use a 28mm wide-angle lens, you will get far more depth of field

than if you use a 200mm telephoto lens set at the same lens aperture and

focused at the same distance; for example, a 24mm lens set at f/8 and focused

10 feet from the subject has greater depth of field than a 200mm lens that is

also set at f/8 and focused at 10 feet. A zoom lens produces more or less the

same depth of field at a certain setting as a fixed-focal-length lens of that same

length; thus, a 28–80mm zoom lens set at 50mm will produce the same depth

of field as a fixed 50mm lens.

You can increase or decrease depth of field by changing any of the above variables,

but keep in mind that they are interrelated. For example, you can

increase depth of field by closing down your lens aperture to a smaller f-stop.

But if you move closer to the subject and refocus, you may actually end up

decreasing the depth of field.

The ability to render your subject uniformly sharp is one of photography’s

great strengths, so most times you will want as much depth of field as the situation

allows. However, there are times when you will want to have the subject

(or another part of the image) sharp and the background or foreground

blurred, such as when you focus on a portrait subject and let the background

go out of focus.