Wednesday, July 4, 2007

Field Guide 101: Exposure - Depth of Field

Misty Morning
Yellowstone National Park, Wyoming 2006

Depth of field is something many beginning photographers struggle with. So first let us define depth of field so there is no confusion. Depth of field is the distance in front of and beyond the plane of focus. There is only one plane in the photograph that is in focus and there is apparent focus toward the camera and beyond the focus plane. Typically the subject will be the on the focus plane. Apparent focus means that more of the image appears to be in focus. For a more detailed description, including the math behind depth of field, visit Wikipedia, Depth of field.

The depth of field is determined by the f-stop. By increasing the f-stop you reduce the aperture diameter and increase the depth of field, which reduces the amount of light. Each stop up doubles the amount of light and reduces the depth of field. It is a good idea to memorize the whole f-stops: f/1.0, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22. I found in the film days I used one-third stops but in the digital world for speed purposes I have now switched to one-half stops. The reason being that with film I needed to be more accurate with my exposure but with digital there is a little bit of fudge factor given the way you want to expose to the right. You still want to get a correct exposure; it’s just that with digital there is more room for error and correction later.

More about exposing to the right later.

Photographers use depth of field for effects like blurred background, known as bokeh, or trying to make the entire scene look as if it is in focus. For an example of an in focus scene take a look at the photo at the top of this article. I used f/16 to get the desired effect. I also was shooting at 45mm. The wider angle the lens the more apparent focus. For this reason, and slow shutter speeds with good tripod use, most landscape photographs appears to be entirely in focus. Telephoto lenses have the opposite problem; very shallow depth of field even at the widest of apertures. Macro lenses suffer from shallow depth of field in spades.

If you were writing a book at this point I would take a couple photos of a flowers illustrating what happens when you open up the f-stop or close it down. Instead I will leave this as an exercise for the reader to do. Use whatever lens you have, put the camera in aperture priority and take the same photo at each whole f-stop. Then go back to your computer and compare the results. If you have more than one lens, or a zoom lens do the same thing at various zoom levels until you understand how the f-stop affects your photograph.

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