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Archive for Sunday, February 26, 2012

Behind the Lens: Depth of field important for focusing on subject

A scene from the Battle of Wilson's Creek 150th Anniversary Reenactment, held near Springfield, Mo., on August 12-14, 2011.

A scene from the Battle of Wilson's Creek 150th Anniversary Reenactment, held near Springfield, Mo., on August 12-14, 2011.

February 26, 2012

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When I photograph a KU football game, even outdoors in daylight, I choose a large aperture (f/2.8, f4.0) for creative purposes.

A typical sunny-day exposure, using a sensor sensitivity setting (ISO) of 200, would be around f/4.0 at 1/2000th. From last week’s column you may recall that smaller f/stop numbers represent larger apertures, which means less depth of field (DOF).

Depth of field is an area in front of and beyond your point of focus that also remains in focus. Why would I want less DOF at a football game? Many photographers do this to place emphasis on one plane of focus, i.e., a runner with the ball. A shallow DOF will render foreground and background subjects less sharp. It’s a creative technique that directs the viewer’s eye to a particular object or subject.

If I were not skilled at photographing football, I could choose an exposure with a smaller aperture to provide greater depth of field. This would make it easier for me to catch a running athlete in focus.

An equivalent exposure of ISO 200 and f4.0 at 1/2000th that would improve my DOF would be ISO 200, f8.0 at 1/500th. My shutter speed is slower but I have gained some significant depth of field by changing my aperture from f4.0 to f8.0. Check out the depth of field calculator at the sitedofmaster.com/dofjs.html.

By entering your camera model, lens focal length, subject distance and your f/stop you can determine depth of field.

Entering the two exposure scenarios above into the chart and using a Canon 5D camera, a 300mm telephoto lens, and a subject 50 feet away, my DOF at f4.0 is 2 feet and at f8.0 about 4 feet. The change in aperture provides an additional zone of relative sharpness from 2 feet in front of to 2 feet beyond my point of focus.

In addition to your aperture, two other things affect your DOF: proximity to your subject and your lens focal length. Here’s what to remember:

• The higher the magnification factor of a lens, the smaller your depth of field. Telephoto lenses will always have less DOF than wide-angle lenses at all f/stops.

• Regardless of what lens you use: the closer you are to a subject, the shallower the DOF; the farther away from the subject, the greater the DOF.

If you like everything to appear sharp in your photos and you photograph wide landscape vistas, architecture, group shots etc., you might prefer smaller apertures (f16, f22) and wide-angle lenses.

If you like to create moody portraits, emphasize still-life objects or isolate athletes in action, you should try using longer telephoto lenses and large apertures (f1.8, f2.8).

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