Focal length
= f/number
Diameter of a lens opening
For example, if a lens has an 8-inch focal length and the diameter of the lens
opening is 2 inches, the aperture is f/4, (fig 2-12).
c. Relationship of apertures.
The maximum opening of a lens can be
reduced by adjusting the camera diaphragm.
This will change the size of the
aperture and, consequently, its light transmitting ability.
If the diaphragm
of the f/4 lens, discussed above, is closed down to 1 inch, the effective lens
aperture is 8 (8 ' 1 = 8). This is expressed as f/8.
d. Full stops.
Full stops are a series of f/numbers, each of which
admits either one-half or twice as much light as the preceding f/number. Table
2-1 indicates the amount of light passed by a lens when set at various stops.
Table 2-1.
Full stops.
For example: A lens set at f/5.6 passes one-sixteenth as much light as a lens
with an aperture of f/1.4. Consequently, a lens at f/1.4 passes sixteen times
more light as a lens with an aperture of f/5.6.
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