Lancaster Rapid Rectigraph - unknown aperture scale

[Matthijs]

Hi all. I've recently purchased a nice looking Lancaster & Sons Rapid Rectigraph 1/2 plate lens. The aperture ring is marked 10 wide open and then closes down by 20, 30, 40 and 50. There is a parallel marking named 'time', starting with 1 (at marking 10) and decreasing in steps of 2, 4, 8, 16, etc.

I've found the conversion table at largeformatphotography.info (here) and I suspect that my lens has the Dallmeyer aperture scale. However, I found this post by Ole, who owns a lens from the same maker, and has determined that it's an actual f/stop scale.

So, which one is it, the Dallmeyer scale or the 'normal' f/stop scale? Before I start measuring and doing math, I hope someone can just give me the answer

[Ole]

I always suspect any old aperture scale to be marked in AN, unless I can prove otherwise.

AN seems to have been one of the most popular scales in Britain for quite a long period - it's my abbreviation for "Arbitrary Numbers".

[Justin Cormack]

Measure the size and focal length and work out the ratio. Mine is surprisingly apparently a real f stop scale with similar markings, though it may just be that the ANs happen to be close enough.