Depth of field - Extending the sharpness

Depth of field of camera photography has to do with front to back sharpness in the scene you are photographing. When you focus your lens on an object, everything at exactly that distance from the camera is in focus, while everything not at that distance is, to some degree, out of focus. Objects that is out of the small depth of field would appeared blur in photography. However, focus doesn't change from sharp to blurry suddenly; it is a smooth transition.

Starting with the point of greatest clarity, sharpness diminishes little by little as distance from this point increases. The gradual change occurs in both directions, in front of the object focused on as well as behind it. Fortunately, the human eye and brain can accept a certain amount of less than perfect focus without paying much attention to it. Eventually, however, limits are reached beyond which sharpness is unsatisfactory. If you measure the distance between the near limit and the far one, you will have what is know as the depth of field.

How will you know what the depth of field will be before taking a picture? Better still, how can you control depth of field to make it suit your own purposes? How to control the zone of sharpness? There are several ways, the three most important involving lens focal length, lens aperture size, and the distance to your subject.

It is important to realize that as lens focal length increase, depth of field gets shallower, all other conditions being equal. The normal lens on a 35mm camera has generous depth under most circumstances. However, the really long lenses, 500mm and over, have such limited depth that focusing must be done very carefully to make sure you have actually selected the exact point you had in mind. On the other hand, wide angle lenses are more and more lavish with their depth as their focal lengths get shorter. With a super wide angle lens depth of field is so extensive that focusing can be dispensed with altogether under some circumstances, such as when the lens is focused at 1 meter (3 feet) and a medium to small f-stop is used.

Lens aperture is the second device for controlling depth of field and is probably the most convenient. Control the aperture to create depth of field. The application is simple: the smaller the stop (all other things being equal), the greater the depth and the larger the stop, the shallower the depth.

The third important factor governing depth of field is focusing distance. With any given lens and f-stop depth can be increased by focusing farther away or decreased by focusing closer to the camera. It is this principle that makes close up photography such a challenge. The very short distances between camera and subject produce extremely limited zones of sharpness.
 

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