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Understanding Depth of Field

Depth of Field Concept
We have all seen pictures in which everything from foreground to background is in sharp focus. In other pictures only the main subject appears sharp, while its surroundings are a soft, out-of-focus wash of color and shape. This effect is to due a concept known as “depth of field.”

The image on the left was shot at f/5.6 @ 1/125 sec., while the one on the right was shot at f/11 @ 1/30 sec.

Circle of Confusion
Although exact focus occurs only at the precise focusing distance, the apparent range of focus can vary considerably. Without going into the physics involved, this is due to an optical phenomena called “circle of least confusion.” When an object is at the exact distance the lens is focused, every point on the object will focus to a point on the film plane. When an object moves out of focus, these points begin to grow and become circles on the film plane. The farther out-of-focus an object is, the larger these circles become (see the top row, below).

CIRCLE OF CONFUSION. The human eye cannot totally resolve focus detail,
so images still appear to still be in focus over a range of distances.

However, up to a certain point, these "circles of confusion" cannot be resolved by the human eye. Our eyes are not capable of discerning them, so the image appears to still be in focus over a larger range of distances and thus has “depth of field” (see the bottom row, above). This apparent area of focus is something the photographer can manipulate by several factors.

Focal Length
You can vary the depth of field from any shooting position simply by your choice of lenses. Long lenses, such as 200mm or more, tend to produce pictures with a shallow depth of field, while short lenses can produce an extensive depth of field.

Subject Distance
The closer you get to the subject, the less depth of field in your pictures. Distant city skylines can have an extensive depth of field. Extreme close-ups, on the other hand, can have an acceptably sharp focus measurable only in the tiniest fractions of inches.

The Effect of Focus
When focusing on a distant subject, focus on a point about a third of the way into the picture to maximize depth of field. This will approximate what is known as the hyperfocal distance, and it works to maximize depth of field for all apertures and focal lengths, although the smaller the aperture and the shorter the focal length - and the greater the distance you shoot at - the greater the depth of field.

The Effect of Aperture
Large apertures (small f-stop numbers) produce a limited depth of field, while small apertures (large f-stop numbers) increase the zone of acceptably sharp focus. The ability to control depth of field is one of the factors that will separate the snapshot shooter from the more advanced photographer. In the two similar photos at the top of the page, the only difference in the shots was the choice of aperture and shutter speed.

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