Help with a meniscus lens f stops

[Rhodes]

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-Brass-Meniscus-Lens-240mm-5x7-8x10-/141225245943?ru=http%3A%2F%2Fwww.ebay.co.uk%2Fsch%2Fi.html%3F_sacat%3D0%26_from%3DR40%26_nkw%3D141225245943%26_rdc%3D1&nma=true&si=ZylRZrdSvXz1cKdS4SDvx5SZe8s%253D&orig_cvip=true&rt=nc&_trksid=p2047675.l2557

[Steven Tribe]

Many meniscus landscapes are as "uncommunicative" as this one!

I would expected the fastest stop to have been more around F.16, but F24 is not impossible, unfortunately! Give us the focal length (comparison with known focal length lenses is probably best) and the diameters of the holes and we will do a caculation for you. Listing this a wetplate lens was a little misleading.

You may be able to detach the wheel and baffle to get faster speeds and less perfect images!

[Andrew]

f11 or f16 are more typical esp for this sort of focal length but a smaller f-stop is possible
I have a small meniscus lens with wheel stops that's about 16.5" and I calculated that one as being f22
[and a tiny meniscus with a focal length of 17" with a fixed aperture that I guessed to be f56 !]

[Jim Galli]

Easy peasy. Measure the hole the lights going through to the film. Then measure from the lens elements to the film plane. Divide. So let's say I measure my aperture as 52mm. Then I measure my bellows extension at 330mm. Divide 52 into 330 and you'll discover you're at f6.3. Especially useful for table top arrangements where you're floating somewhere way off infinity approaching 1:1. Math is the same. No bellows factor calcs, etc. Just the hole diameter into the extension to the film plane.

[Rhodes]

Thank you for the replies. I do not have the correct focal length. As the seller says, it may around 240mm, have to calculate the correct focal length! My calliper is not a great one, but the larger aperture is 11mm (I think).
Will try as Jim said also!

[Emmanuel BIGLER]

Hello from France !
Well, there should actually be little ambiguity finding the focal length of a thin, single lens element, where the principal planes are located very close to the lens vertices.
However for the f-number: actually it depends where, and how far from the lens element the waterhouse stop is located.

- if the f-stop is located IN FRONT of the single lens element, like in Wollastton's meniscus lens, then no problem, the entrance pupil IS the f-stop itself. Hence the f-number, defined, in general, with respect to the diameter of the entrance pupil and not with respect to the physical iris diameter, is actually f/D where D is the physical iris diameter and f the focal length.

- however if the waterhouse stop is located at a certain distance BEHIND the lens element, the true, photometric, f-number is slightly different. Actually, the diameter of the entrance pupil, as seen through the lens element, from the front of the whole lens, is different from the physical iris diameter. But since the waterhouse stop, in any case, is located quite close to the glass, a small fraction of the focal length (here: 240 mm) both diameters are not very different, and there is a reasonable probability that f/D yields an acceptable value for re-computing a useable f-stop scale.


My caliper is not a great one

In any case you do not need a very precise instrument.
If you assume that your focal length is perfectly known, and that an error of 1/3 of a f-stop is acceptable in recomputing the f-stop scale, it means that you are allowed to measure 125% instead of 100% of the diameter ! For a hole of 11mm in diameter, 25% of error is about 3 mm... your caliper is certainly much better than that ;)
And even if, in a worst-case scenario, we are wrong by 25% on the f-number, for example you can be wrong by 12% on the focal length and 12% on the f-stop diameter, it means an error of 30 mm on the focal length and 1.5 mm on the diameter ...
You can sleep quietly before any shooting session: your caliper will be a perfect measuring instrument for your purpose !


Have fun with this vintage lens !