For a telescope, the diameter of the exit pupil can be calculated by dividing the focal length of the eyepiece by the focal ratio ( f-number ) of the telescope.
22.
:It's actually quite difficult to make a'fast'Newtonian reflector & ndash; that is, one with a small focal ratio that will fit in a short tube & ndash; that produces good images.
23.
In photography the focal ratio varies the focal-plane illuminance ( or optical power per unit area in the image ) and is used to control variables such as depth of field.
24.
The effective aperture is expressed by an f-number or f-stop ( derived from " focal ratio " ), which is proportional to the ratio of the focal length to the diameter of the aperture.
25.
The maximum usable aperture of a lens is specified as the focal ratio or f-number, defined as the lens's focal length divided by the effective aperture ( or entrance pupil ), a dimensionless number.
26.
In addition, the telescope allows very high-quality infrared observations due to the advanced protected silver coating of its mirrors, its small secondary mirror ( due to the f16 focal ratio ), and advanced ventilation systems.
27.
Numerically large focal ratio slower ( bigger number ) than f / 12 is generally considered slow, and any telescope with a focal ratio faster ( smaller number ) than f / 6, is considered fast.
28.
Numerically large focal ratio slower ( bigger number ) than f / 12 is generally considered slow, and any telescope with a focal ratio faster ( smaller number ) than f / 6, is considered fast.
29.
So somehow they saw such an unfocused rainbow blur of colors with sane focal ratios but no one thought of using colored eyepiece glass or something ? talk ) 20 : 00, 7 June 2012 ( UTC)
30.
As with any lens, increasing the focal ratio of the lens increases depth of field by reducing the level of blur of out-of-focus objects ( compare the effect of aperture on depth of field in photography ).
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