controlling depth of field

a 9 leaf iris diaphragm c1970

Toronto. As a general rule, the longer the focal length of a lens, the smaller its maximum possible f/stop and the less its depth of field. Conversely, the shorter the focal length, the greater the depth of field and the larger its maximum possible f/stop can be. The human eye has a relatively short focal length and a very wide depth of field making almost everything you see in focus – especially in sunlight when the iris closes to help limit the light intensity hitting the retina.

Smartphone cameras also have short focal lengths. My older smartphone has a camera lens of 3.3mm f/2.4 (equivalent to a 35mm lens in angle of view). Newer models have a lens equivalent to a 28mm lens and its wider angle of view and even greater depth of field.

Once photographic lenses increased in speed, a fixed Waterhouse stop was generally used to increase the depth of field (and reduce the light hitting the sensitive media). A slot in the lens barrel was used to slip the Waterhouse stop between the lens elements. Different Waterhouse stops were made available to adjust depth of field.

When iris diaphragms became common place, Waterhouse stops disappeared and the issue was the number of leaves used. The more leaves that were used, the more expensive the diaphragm was to make and the rounder the tiny aperture became. Aperture shape  affected the bokeh of the lens.

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