Tuesday, November 27, 2012
In Search Of Sharpness
How to take control of depth of field in-camera and in the computer
Another technique to maximize DOF is tilting the front optic to induce the Scheimpflug principle. Normally, the zone of focus is parallel to the aligned front lens and the film or sensor, but if the axis of the lens is tilted, the zone of focus tilts with the new axis. Imagine, for example, that you're photographing a field of flowers. Normally, the plane of focus is vertical, ranging from the ground, up the stems of the flowers and straight into the air above them. Tilting the plane of focus to skim across the tops of the flowers and into the distance applies the available DOF to the area of the image that's most important. Today's tilt/shift lenses work on this principle, and large-format view cameras have been using the front tilt since glass plates and sheet film were invented.
For each lens and ƒ-stop combination, there's a point—the hyperfocal distance—from which all objects will be sharp into infinity. The greater the focal length and the larger the aperture, the farther away the closest focus point will be. Charts are readily available to assist in these calculations. Some prime wide-angle lenses actually have DOF markings on the lens barrel to make the setting easier to attain. By placing the marking for a certain ƒ-stop on infinity and reading its corresponding marking on the focus indicator, you'll have set an approximate hyperfocal distance for these lenses.
A prime example of a useful hyperfocal distance calculation is a wide-angle lens of 16mm set to ƒ/16: the area from one foot in front of the lens to infinity will be sharp. The lens focus indicator must be set to two feet for this to work. Think flowers in the foreground and majestic mountains in the distance. To attain this maximum hyperfocal distance for a 105mm lens at ƒ/16 would require the photographer to set the lens focus indicator to 88 feet, and everything from 44 feet to infinity would be sharp. To take this to an extreme, my 500mm telephoto lens at ƒ/16 needs to be set at 1710 feet to get everything from 855 feet to infinity sharp. (Good luck in finding a setting for 1710 feet on the lens.) I carry a hyperfocal chart in my camera bag to assist in setting my focus point with various lenses and ƒ-stops. If you use an iPhone, check out the Focalc, DOFMaster or DoF Calculator apps; there are many more for Apple and Android phones. Just type in your camera, lens and ƒ-stop, and the app tells you the focus distance setting and what the nearest sharpness will be.
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