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Setting the aperture
To preset the lens aperture, turn the knurled aperture ring on the lens barrel
until the desired f/number lines up with the black dot in the top center of the
milled ring. The diaphragm can also be preset for intermediate apertures
between the click-stopped markings.
Aperture and Depth of Field
The lens aperture determines the amount of light which passes through the lens
and strikes the film. It also influences depth of field. Apertures are expressed
in f/numbers, with larger numbers representing smaller apertures. For
example, f/4 is larger than f/8 and allows more light to reach the film. Each
f/number admits exactly twice as much light as the next largest number, so
when the aperture is increased or decreased by one stop, the exposure time
required is doubled or halved respectively. For example:
Aperture f/1.4 f/2 f/2.8 f/4 f/5.6
Relative Exposure Time (seconds) 1/500 1/250 1/125 1/60 1/30
Depth of field refers to a zone extending in front of and behind the plane of
sharpest focus. Within this zone, blur (or unsharpness of the image) will be
negligible and everything can be accepted as being in sharp focus. It extends a
greater distance behind the subject in focus than in front. Depth of field
depends on three factors: focal length of the lens, lens-to-subject distance and
taking aperture. The smaller the aperture and the shorter the focal length of the
lens, the greater the depth of field (for example, wideangle lenses have more
depth of field than telephotos). Also, the closer the subject, the smaller the
depth of field. These three factors can be adjusted independently or in
combination to give the photographer creative control over the final picture.
By stopping down the lens only, the depth of field can be increased, as
illustrated by the following three photographs:
1.Lens at f/4. Small depth of field with only main subject in focus.