# f-number

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Related to f-number: focal length, Numerical aperture

## f-number

, f number
Photog the numerical value of the relative aperture. If the relative aperture is f8, 8 is the f-number and indicates that the focal length of the lens is 8 times the size of the lens aperture
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005

## f-stop

(Focal-STOP) The f-stop is the "aperture" opening of a camera lens, which allows light to come in. It also determines how much is in focus in front of and behind the subject (see depth of field). The f-stop is one of the two primary measurements of a camera lens. The other is the "focal length," which establishes how much of the scene is in view (see focal length).

Length Divided by Opening
The f-stop is the focal length of the lens divided by the diameter of its opening. Each consecutive f-stop halves the opening of the previous. For example, an 80mm lens with its f-stop set to f8 means that the optics inside the lens create a diameter equivalent to 10mm. Changing the f-stop to f16 creates a diameter of 5mm. See focal length.
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In order to change an EV, after an f-number is fixed to f/5.6, a shutter speed is adjusted for taking test images.
8 (a) Focal length 35mm 28mm f-number f/5.6 f/5.6 Shutter speed 1/8s 1/4s (b) Focal length 35mm 28mm f-number f/5.6 f/5.6 Shutter speed 1/15s 1/8s (c) Focal length 35mm 28mm f-number f/5.6 f/5.6 Shutter speed 1/30s 1/15s (d) Focal length 35mm 28mm f-number f/5.6 f/5.6 Shutter speed 1/60s 1/30s (e) Focal length 35mm 28mm f-number f/5.6 f/5.6 Shutter speed 1/125s 1/60s Table 2.
For a given solar concentration ratio, what f-number makes the cooker most tolerant to misalignment?
insignis (2.16-2.97) is far less than that of the nocturnally active web building uloborids, whose f-number ranges from 0.88-1.70 (Opell & Ware 1987).
The best theoretical f-number is equal to the focal length (FL) divided by the entrance lens diameter (d).
The f-number simply is not a valid measure of lens quality.
The image entitled Greater Flamingos during sunrise at Askar coast, Bahrain, was snagged using his Canon EOS 1DX Mark II with a Canon EF 400mm f/2.8L + 2X extender, a shutter speed of 1/1000, an F5.6 F-number and film speed (ISO) of 800.
A simple way to adjust this is to use a keyboard shortcut, which is an available function on most laptops, (usually the Fn button, plus one of the F-number keys on the top row).

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