Understanding Depth of Field

Changing the aperture number of a camera does two things:

• It controls the amount of light entering the camera.

• It determines the amount of Depth of Field (D of F) in the picture.

Although a lens can focus at only one distance, the decrease in sharpness is gradual both in front of and behind the focused distance, so that within the D of F, the un-sharpness is unnoticeable. This distance is approximatley one-third in front of the point of sharpest focus and two-thirds behind the point of sharpest focus.

FIG. 1.12 Depth of field.

For some pictures, such as underwater wide angles, a large D of F is appropriate, while for others opportunities, such as fish portraits, a small D of F may be more effective.

Small apertures like f22 provide the greatest D of F, while a large aperture such as f2.8 provides the narrowest D of F.

Compare the difference in D of F between Fig. 1.13 and Fig. 1.14.

The shorter the distance between the lens and the subject the smaller the depth of field. Whilst the greater the distance between lens and subject, the greater the depth of field.

• If you want a large D of F with both foreground and background in focus, use a small aperture such as f11, f16, f22.

• If you want a narrow D of F with only a shallow focus, use a large aperture such as f2.8, f4 or f5.6.

After a little practice you will develop a feel for the effect for both sharp and out-of-focus portions of your photograph.

Depth of field -

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FIG. 1.13 This close-up study of a clam was taken with a wide aperture of ß.5 (small D of F) to emphasise detail and sharpness within the siphon compared with the pattern detail in the background. Nikon D200 Nikon 60 mm macro lens, ISO 100, two Inon 220z flashguns.

FIG. 1.14 Taken of subject at f22, 1/180th sec with a Nikon 105 mm macro lens. Notice how the subject is sharp throughout as it extends into the background.

FIG. 1.15 Taken at the same time but with a wider aperture of f5.6 at 1/180. You can see how the D of F is significantly reduced, which has the effect of making the facial features of the subject stand out more.

FIG. 1.16 Typical wide angle scenic with the familiar characteristics of a huge depth of field. Tokina 10 17 mm at the 10 mm end, f8 at 1/250th sec, ISO 200, twin Inon 220z flashguns.

FIG. 1.17 Such is the D of F of wide lenses, some users pre-set the focus at about 2 m knowing that everything from 1 m to infinity will be in sharp focus. I locked off the focus of my 10.5 mm fisheye at about 2 m, confident that this classic wide angle of the Red Sea wreck Giannis D would be sharp throughout.

Nikon D200, f67 at 1/45th sec, ISO 100.

FIG. 1.17 Such is the D of F of wide lenses, some users pre-set the focus at about 2 m knowing that everything from 1 m to infinity will be in sharp focus. I locked off the focus of my 10.5 mm fisheye at about 2 m, confident that this classic wide angle of the Red Sea wreck Giannis D would be sharp throughout.

Nikon D200, f67 at 1/45th sec, ISO 100.

In these situations you're better taking the shot with a higher ISO than not taking the shot at all!

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