I recently picked up a J. Lancaster & Son meniscus lens from Eddie here. Its aperture scales are unlike any other I've encountered before. There are two scales on the lens. One is marked 10, 20, 30. A second is labeled "TIME" and is marked in the increment 1, 2, 4, 8--with the number 1 corresponding to "10" on the other scale, 4 corresponding to "20", and 8 corresponding to "30". The TIME scale is different from similarly numberd US aperture scale though (where 1 = f/4, 2 = f/5.6, 4 = f/8, etc.) Based on my own calculations, it looks like the lens is somewhere in the area of f/11-f/16 at its widest aperture and f/22-f/32 at its smallest opening. I was wondering if there's some sort of published conversion table for what looks to be J. Lancaster & Son's proprietary scales.
Oh, and unrelated to that, does anyone with a copy of the Vade Mecum know in what range of focal lengths the J. Lancaster & Son meniscus lenses were sold?
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