J. Lancaster & Son meniscus lens questions

[benrains]

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?

[CCHarrison]

Here is an 1888 ad that may help

Dan

[Stephane]

10 = f/10
20 = f/20...
no conversion needed.
the numbers 1, 2, are time equivalent for smaller apertures if you measure your base exposure from f/10. Thats at least how I saw it.

[benrains]

10 = f/10
20 = f/20...
no conversion needed.
the numbers 1, 2, are time equivalent for smaller apertures if you measure your base exposure from f/10. Thats at least how I saw it.

Ah, clever! That makes sense.

[Steven Tribe]

On larger objectives the 1,2,3 etc. scale is actually labeled "time". This very common for Lancaster lenses of all types.

[Ole Tjugen]

The "10 - 20 - 30" and so on scale is usually marked in AU on older J. Lancaster lenses.

"AU" is an abbreviation for "Arbitrary Units". They bear little or no resemblance to "real" f-stops. At least that's what I've found by looking hard at my own!

[Jim Andrada]

I found that if I assumed that 10 was f/11 and 20 f/16 and 30 f/22 and as far as the iris would close down f/32, the exposures worked out pretty well.