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Öffnungsverhältnis n
comp., MS	focal ratio (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))
earth.sc., life.sc.	relative aperture of a lens
earth.sc., met.	focussing lens aperture
life.sc.	relative aperture
opt.	relative aperture (the ratio of the focal length to the entrance pupil diameter); f-number (if is a measure of the light-gathering ability of a lens); focal ratio; focal-length ratio
phys.	lens aperture ratio (e.g. of the objective)
tech.	aperture ratio

Öffnungsverhältnis: 2 phrases in 2 subjects

Optics branch of physics	1
Physics	1