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Colin Myers
16-Feb-2007, 11:48
Hello, I wonder if anyone can assist with this. I have just acquired a Voigtlaender Euryscop series 1V, #3 lens. I understand this to have a focal length of 11.5" and it has one original, full aperture stop. Measuring the hole in this stop and doing the math, gives an answer of 6.5. Reading up on this in old archives , it should be an f6 lens, perhaps not enough difference to worry about! If I am trying to replicate the original set of stops, how many should there be for this lens, and what sort of ratio did they stop down to? I would be most grateful for any help or advice. Colin Myers

Scott Davis
16-Feb-2007, 14:13
I'd send it off to SK Grimes to have them make you a set of custom stops for the lens. They've done this many times before and will probably do a better job of it than most of us DIY types can.

Ernest Purdum
16-Feb-2007, 14:39
I am guessing that there were probably about six stops oriinally, but you aren't limited to what was originally supplied. Consider how far you might want to stop down and go from there. Next down from f6 is f8, then f11, f16, f22, f32 - do you want to go further?

Paul Fitzgerald
16-Feb-2007, 19:21
Collin,

" Measuring the hole in this stop and doing the math, gives an answer of 6.5. Reading up on this in old archives , it should be an f6 lens, perhaps not enough difference to worry about!"

That would make sense because of the optical illusion of the lens, they measured the 'apparent' aperture that the film sees through the lens. Measure throught the lens from the rear to a transparent scale in front and see the difference.

Have fun with it.

Colin Myers
17-Feb-2007, 08:55
Thanks all of you. I tried the viewing from the rear and reading a transparent rule. Now almost spot on f6. Many thanks for the tip. Colin

C. D. Keth
17-Feb-2007, 09:07
What do you want to use the lens for? If it's portraits, I wouldn't bother going further down than 22. If you want to do some landscapes with it, I would go to 64 or even perhaps 90.

Paul Fitzgerald
17-Feb-2007, 09:09
Colin,

Now you can cheat. Multiply the diameter of the stop you have X 6 to get the 'magic' number, then divide the 'magic' number by 8, 11, 16, 22, 32 for the proper sizes for the other stops. They will have the same difference ratio. I have a euryscop IV #4 and it raises the contrast and detail stopping down to f/11, it's marked 6, 8, 11, 16, 22, 32 for the iris. nice look to it.

Have fun.

Colin Myers
17-Feb-2007, 12:10
Paul,
Thanks for that. I had already worked them out longhand, using the actual stop diameter, with obviously, very slightly different results. But this is a lot quicker. Neat tip!

Colin Myers
17-Feb-2007, 13:29
Paul,
Fired up with enthusiasm for the "quick" method, I thought I would try and apply this to another Euryscop I have. I think this is a series 4, #6, focal length 21.33". It came complete, with what I believe to be a set of 6 original waterhouse stops. Using the
sighting from the rear and transparent ruler ploy, this did indeed give f6 as the aperture of the largest stop. This looked promising, so mutiplying this figure by 6, then dividing by 8, 11, 16, 22 and 32 provided some numbers. I compared these figures with both actual measured diamaters of the stops, then in desperation, posted them into the lens barrel and took the optical measurement of each. All quite different results. Did they use a different range of part f stops back in the 1880's?
I then calculated the ratio of the actual measured circle areas to one another . Comparing the largest hole to the second largest etc, gives 71% or 1.4, 58% or 1.7,47% or 2.1,49% or 2.04 and 56% or 1.78? Any ideas?

Paul Fitzgerald
17-Feb-2007, 19:15
Hi Colin,

Yes, there have been quite a few different stop systems in the past, the 1.4 system used today is the newest standard, from about 1912. Before that was the US system (1, 2, 4, 8, 16, 32, 64, ect), before that a few from each country, but I would have thought they would be standardized to themselves.

You could try a light meter to the GG, but F/stops are not T/stops. Close but not quite the same.

All of my Voigtlander lenses have irises but I did use the 'magic number' method to make stops for a Goerz Lynkeioskop and a B&L tessar. It does work out measurement wise, DOF wise and lightmeter wise. The bear is getting the first stop measurement right, but you already have full stops to measure.

Yes, I know most people measure modern lenses from the front. I guess backasswards is the way Voigtlander did it in the 1800's through to Heliars.

Good luck with it.

Ernest Purdum
18-Feb-2007, 11:05
Although Paul Fitzgerald is right in saying T-stops are different from f-stops, it could be argued that they are better, particularly with old uncoated lenses. I have the results of some earlytesting of aerial photogrammetry lens transmission showing some very large variations between different lens types.