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Blendenzahl f
astr.	f-ratio; f-number
comp., MS	f-number (In photography, the diameter of the aperture expressed in terms of the focal length of the camera lens. The greater the f-number, the less light reaches the film and the more in focus the objects in the foreground and the background will be. For example, with an f-number = 16 the objects in the far background will be in focus. Whereas, with an f-number = 2 the background will be blurry (while foreground objects are in focus))
opt.	stop number (a measure of the light-gathering power of the aperture; the ratio of the focal length to the diameter of the largest entering parallel beam that can pass through the lens); stop value (a measure of the light-gathering power of the aperture; the ratio of the focal length to the diameter of the largest entering parallel beam that can pass through the lens)
phys.	focal ratio; stop number (of the camera lens)

Blendenzahl: 3 phrases in 3 subjects

Earth sciences	1
Microelectronics	1
Optics branch of physics	1