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Tuesday, December 8, 2009

Maximize Depth of Field for Ultimate Sharpness

There are really two kinds of sharpness in a photograph. One is the sharpness of your main subject, say, a person standing on the beach and that sharpness depends on how steady you were at holding the camera, the shutter speed you used, how carefully you focused on your subject and how still your subject was standing. If you are careful with your technique, your subject should be quite sharply focused.

The other is depth of field or "near-to-far" sharpness and that describes how much of the beach in front of and behind your subject is also in sharp focus. Depth of field sharpness is not an absolute thing that has exact starting and stopping points, rather it's a zone of what is called "acceptable" sharpness. Ultimately how sharp a photo appears also depends on where you're seeing it and how far away you are from the image. If you're looking at an online version it will often look sharper than it will in a print, largely because a print is made of paper and ink and has a surface that can interrupt our interpretation of sharpness. Also, images online are rear-illuminated so they will always look brighter and, within reason, sharper. And if you're looking at a print, the farther you are from the print, the sharper it will appear (particularly if you refuse to wear your reading glasses).

But you can maximize depth of field, that zone of acceptable sharpness, if you know what factors affect it. Basically three things (in combination) determine how much depth of field sharpness an image will have: lens focal length, your distance from the subject and the aperture you're using. Here are some things to keep in mind when you're trying to maximize depth of field:

  • Wider lenses have inherently more depth of field. That means that for a given f/stop at a fixed lens-to-subject distance, the shorter the focal length of the lens, the more that will be in sharp focus. Wide-angle lenses provide the most depth of field and telephoto lenses the least.
  • With any given lens, the smaller the aperture (f/16 as opposed to f/4, for example) the more depth of field you'll get.
  • With any given lens at any given f/stop, the farther you are from your subject, the more depth of field you will have. If you were shooting a picture of a friend with a 28mm wide-angle lens at f/8, for example, your photo would have inherently more depth of field if you stood 10 feet away than if you stood five feet awy.
Remember, it's these three factors in combination that really determine depth of field. If you're photographing a landscape and want lots of near-to-far sharpness, choose a wide lens and a small aperture and you will increase depth of field. I shot this photo of Las Olas Boulevard in Fort Lauderdale using a combination of a wide-angle lens and a very small (f/20) aperture to wring the maximum amount of sharpness from the image. Because you're usually using small aperture to get a lot of depth of field, you'll also be using relatively slow shutter speeds. In order to be able to shoot this photo at such a small aperture, I had to reduce the shutter speed to 1/30 second. You can see the motion blur caused by the slow shutter speed in the pick up truck. Using a tripod helps to steady the camera, but you'll still get blur with any moving subjects.

Of course, there will be times when you want to minimize depth of field--to limit sharpness in a portrait, for example, and to do that...you'll have to read the next tip!

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