f number

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f number

Designation for a photographic lens which gives the ratio of the focal length to the diameter of the effective aperture or entrance pupil. Example: f/8 means that the lens has a focal length eight times the diameter of the entrance pupil. Syn. f/stop; f-value; focal ratio; lens speed. See relative aperture.
Table F1 Sequence of f numbers used in photography with the corresponding relative image brightness and exposure time to maintain constant film exposure. The relative image brightness is equal to the square of the f number fraction
f numberfocal length/pupil exact
f number
diameterrelative image brightnessrelative exposure time
f/0.50.5f/0.5001/0.250.12
f/0.70.7f/0.7071/0.50.25
f/11f/1.0001/10.5
f/1.41.4f/1.4141/21
f/22f/2.0001/42
f/2.82.8f/2.8281/84
f/44f/4.0001/168
f/5.65.6f/5.6571/3216
f/88f/8.0001/6432
f/1111f/11.3141/12864
f/1616f/16.0001/256128
f/2222f/22.6271/512256
f/3232f/32.0001/1024512
References in periodicals archive ?
Dr Arditti explained that the HyperStar system was a means of reconfiguring widely available Schmidt-Cassegrain telescopes (SCTs), which typically had focal ratios of around f/10, into Schmidt camera configurations with much faster focal ratios of around f/2.
Unfortunately, although Baby's eyepieces have the same focal lengths as Mama's and Papa's, they are Huygenians, an inexpensive design that works particularly poorly in scopes with short focal ratios.
Apochromatic refractors with short focal ratios offer unparalleled wide-field performance, a characteristic valued highly by many deep-sky observers and astrophotographers.
Manufacturer Celestron Discovery Telescopes Model NexStar 80GT 6" f/5 DHQ Price (US$) $319 $339 Optics type refractor reflector (achromat) (Newtonian) Aperture 80 mm 152 mm Focal length 400 mm 762 mm Focal ratio f/5 f/5 Finder reflex 6 x 30 Focuser size 1.
The focal ratio is a geometric indicator of how much light is delivered to the detector (be it a digital sensor or your eyeball) per unit area of the focused image, and a key point when selecting a camera later.
One last bit of essential mathematics for telescope users is the focal ratio (f/ratio)--the ratio of focal length to aperture.
ASTROPHOTOGRAPHERS pursuing deep-sky quarry often desire a generous focal length, which produces enough image scale to resolve small details in their targets, but would prefer not to sacrifice focal ratio to get it.
price range $299 to $399 Manufacturer Celestron Orion Telescopes Model Firstscope 80EQ Explorer 90AZ Price (US$) $299 $299 Optics type refractor refractor Optics subtype achromat achromat Aperture 80 mm 90 mm Focal length 900 mm 910 mm Focal ratio f/11 f/10 Finder reflex 6 x 26 erect Focuser size 1.
0-mm focal length, f/9 focal ratio, ISO 100, and V2-second exposure.
7 focal ratio that places the focuser at eye level when the scope is aimed at the zenith.
The focal ratio or f/number of a telescope is also important, but not for the reason camera buffs might think.