I need help to work out the aperture settings for a vintage lens, believed to be approx 1915. The lens is marked:
H Duplouich, Paris, Rectilicne Extra Rapid
That is all: no max aperture and no focal length (my guess for the latter is 100mm).
The aperture scale runs from 2 to 22 with equidistant segments. 2 is the smallest aperture, 22 is WIDE OPEN, so it is not related to an f-stop scale. I think it is the aperture diameter in mm.
An idea how to work out f-stops?
@tapasinthesun @rdm focal length ‘f’ is the distance from the aperture iris to a sheet of paper when the wide-open lens casts an in-focus (upside-down) image of a distant object on the paper.
F-stops are literally a fraction. f/8 means “aperture diameter is focal length divided by 8”
The relationship between focal-length, stop number and diameter is d=f/n or f=nd or n=f/d
So with f=100 and d=5mm that’s n=20 (d=f/20)
f=100 and d=22 gives you f/4.5
ETA: the reason we use stop numbers rather than diameters is that they’re comparable between lenses. f/8 on a 100mm lens gives you the same exposure as f/8 on a 25mm lens. The ratio between adjacent stop number is such that the area doubles at each stop, which means if you open up one stop and halve the shutter speed at the same time, the exposure is equivalent; you no longer need to do math to set exposure, just count clicks.
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