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Depth of Field and Aperture Priority

Depth of field in photography refers to a range of distance that appears sharp (or in focus) to the eye of the viewer. Depth of field will vary considerably depending of the camera being used, the focal distance, lens quality and type, and most importantly, the aperture setting. The aperture setting refers to how much light is being let in to the camera (onto the film or digital sensor) within a given time period (the shutter speed.)

We’ve seen those images where the subject is tack sharp then fades lightly to a total blurred background. The light will often blur into lovely specular highlights and reflections known as Bokeh. Bokeh is seen as circles of varying light in the blurred areas of the photograph. This effect can force the viewer’s eye into an intended focal point: generally, but not always the primary subject. This is known as a shallow depth of field.

A shallow depth of field (where only a small portion of the image is sharp) is achieved by opening the aperture to its widest setting, thus letting in more light. Some cameras have aperture settings as wide as f/1.2, however f/2.8 – f/5.6 are common settings to achieve maximum blur in the non-focal areas.

A maximum depth of field (where the image is sharp front to back) is achieved by “stopping” down the aperture to let in less light. As is the case with those Ansel Adams images where you can see the moss on a rock in the foreground as clearly as the mountain tops many miles away. He achieved this with a maximum depth of field, in his case as small as f/64. Most cameras today will generally stop down to f/22 and possibly f/32.

Try these experiments:

As an exercise in maximum depth of field, set your manual camera on the aperture priority setting. You want to achieve a “front to back; sharp as a tack” image. Adjust your aperture to the smallest opening, somewhere around f/22. The camera will set it’s own shutter speed, which you don’t need to worry about for this exercise. If your camera doesn’t have an aperture priority setting, you will need to get and use a light meter for this exercise.

As an exercise in shallow depth of field, keep your manual camera set on aperture priority. Adjust the aperture to the largest opening you have, somewhere around f/5.6 or smaller. Again the camera will set it’s own shutter speed. If the lens on your camera is of good quality and your aperture opening is large enough, you may obtain the mysterious bokeh or aesthetic quality of blur.

Keep in mind only cameras with manual settings can be adjusted to change the aperture opening or f/stop. If you own a point and shoot camera but want to achieve creative depth of field, you may be able to trick your camera. When shooting in portrait mode your fully automatic camera will choose a wider aperture (shallow depth of field) to ensure the subject is in focus but the background is blurred. When shooting in landscape mode the same camera will select a smaller aperture to ensure the entire scene is in focus.

Using depth of field is an effective and fun way to capture the exact image you want. Keep your heart in your craft and, as always, challenge yourself to try new things and grow as an artist.

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