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:
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Aperture
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f/1.4
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f/2
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f/2.8
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f/4
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f/5.6
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Relative Exposure Time (seconds)
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1/500
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1/250
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1/125
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1/60
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1/30
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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.
  
2.Lens further stopped down to f/8. Larger depth of field.
 
3.Lens at smallest aperture. Creat depth of field with subject, background and
foreground in focus.
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Using the Depth-of-Field Preview Button
The depth of field at taking aperture can be seen by depressing the depth-of-field
preview button located on the front of the camera. When the button is pressed, the
diaphragm closes down to the preselected taking aperture and the exact conditions
of taking aperture and allows you to see how much background or foreground is in
or out of focus. Depth of field can also be determined by referring to the color-coded
scale printed on the lens barrel. The colored lines on either side of the main index
mark (black arrow) correspond to different f/numbers which are engraved in corresponding
colors. With the lens focused on a subject and set for a preselected aperture, depth
of field can be read from the distance scale. For example, with the 50mm f/1.4 lens
focused at 15 feet (4.5m) and used at a taking aperture of f/16, depth of field extends
from approximately 8 feet ( 2.4m) to infinity.
caution:
Do not release the shutter while the depth-of-field preview hutton is held down as
this may result in the reflex mirror remaining in the locked-up position. If this
should happen, the mirror will not return to its original focusing and viewing position
until after one extra exposure has been made.
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