English term (edited): setting the aperture on a ring in third-stop clicks

the ring on the objective has "clicks" (selectable positions) calibrated in such a way that after 3 clicks you double the amount of light passing through the lenses, so moving one "click" is one third of a jump to "doubling" the light (same principle is applied for the exposure time, or the sensitivity of the film - all done in "3 steps = doubling the value", so 1 click up for the aperture can be compensated by 1 click down in film sensitivity or in exposure time)

https://www.google.co.uk/search?q=réglage du diaphragme et d...

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Note added at 1 hr (2017-08-19 19:52:13 GMT)
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Most digital cameras are calibrated in the same way: you can correct the exposure by steps in such a way that 3 steps up = +1 = doubling the incoming light or 3 steps down = -1 = halving the incoming light

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Note added at 1 day18 hrs (2017-08-21 12:56:31 GMT)
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to make it clearer:

for an hypothetical objective on which you could open/close the diaphragm between F32 and and F2 (i.e. letting through between 1/32 and 1/2 of all incoming light) you would have a range of 4 full steps - you could double the incoming light 4 time:

you could double the light from 1/32 to 1/16, from 1/16 to 1/8, from 1/8 to 1/4 and from 1/4 to 1/2

for EACH of these full steps (each equivalent to "doubling the light" once) you would have 3 stops spaced in "third-stop clicks"

so this (not so) hypothetical objective would have 4x3 = 12 "third-stop clicks" covering the full range of possible opening of the diaphragm