I bought a few time ago a unnamed brass meniscus lens, the focal length is about 240mm (I think, since there is no marks on the lens). The f/stops are in a revolving disk, the trouble is, no marks of any kind so, I do not know what are the apertures.
I tried calculation the F stops trough this: N= f/D (Wikipedia source). If I am doing right and measuring the diameter of the apertures also right, the results that I got were this: f/24; f/30; f/40 and F/120. Can this be right?
I leave the link for the lens: http://www.ebay.co.uk/itm/Unnamed-Br...p2047675.l2557