Re: Volute aperture scale
Are you sure that is US? Everything I've ever seen in US goes from 8 to 128.
What an odd assortment of numbers. I suppose interpolation is the way to go: 16 is somewhere between 13.6 and 19. ;)
Re: Volute aperture scale
I can answer part of the question. There are three scales because one is for both lens elements together, one for the rear element used singly behind the shutter, and the third for the front element used singly behind the shutter. The first is the outside scale, the third the inside scale.
I can find no reference to this particular f/stop scale in my files, but this can't be the only shutter ever manufactured with these numbers.
Re: Volute aperture scale
BrianShaw - Actually, I'm not sure anymore if it is US or not. The numbers are certainly a riddle ....
Peter - Thanks ! I never occurred to me to think of the scales the way you explained it, I've seen some old catalogs that
mention three sets of lenses as a kit for this type shutter, so I assumed that there was an missing cell and one of the scales was
for that.
Re: Volute aperture scale
It looks like what they did was start from the wide-open f/# and mark out the stops from there. For example, for the one that starts at 6.8 the next stop would be 6.8 * sqrt(2) ~ 9.6 and the next 6.8X2= 13.6. On a modern version I suppose there would still be a mark for f/6.8 (wide open) but then they would mark it in the usual sequence for f/8, f/11, etc.
Re: Volute aperture scale
What bentlyR said. Each scale doubles every other number, indicating the f/stop progression. F/6.8 is a fairly common starting point, (Gundlach's Turner Reich was an f/7; this might be an early version of that lens), and the other scales start where their single elements convert to.
Re: Volute aperture scale
The doubling of every other number I understand for the scale that is on the shutter, but how does that convert to modern f stops ?
Do assume that f6.8 is in between f 5.6 and f8 and proceed from there ?
I'm not understanding bentlyR's formula - 6.8 times the square root of what ?
I'm a bit advanced math challenged :o
Re: Volute aperture scale
This might be a "good enough" approximation of your f-stop scale to a modern f-stop scale:
6.8 = 8
9.6 = 11
13.6 = 16
19 = 32
72 = 64
Re: Volute aperture scale
Quote:
Originally Posted by
BrianShaw
This might be a "good enough" approximation of your f-stop scale to a modern f-stop scale:
6.8 = 8
9.6 = 11
13.6 = 16
19 = 32
72 = 64
How did you get those f stop values ?
Re: Volute aperture scale
Basically, they're saying that f/6.8 is half-way between f/5.6 and f/8.
F/numbers double by the square root of two. 1, 1.4, 2, 2.8, etc. each larger number is the preceding number multiplied by the square root of two (and rounded a bit). Your shutter is marked with f/numbers that multiply each previous number by the square root of two, but it starts with a different number. 6.8xsquare root of two is 9.6, so that's a full stop.
Most shutters aren't marked on the half-stop because you can't fit all the numbers on the darned scale. I'd just read them as 5.6 1/2, 8 1/2, 11 1/2, etc. so they work a little more easily with your light meter.