Old brass lens question

[Nitesail]

Hello, hoping someone can help. I have an old brass lens from around 1895 and I have attached a pic of its description from an old catalog. My problem is that it uses 0-5, all spaced equally, as the “stops”, AND it’s a convertible. Does the minimum, wide-open f-stop change as I make the lens “longer” or “shorter”? All of the screw-on front and rear lens cells are the same thickness. Is “0” the same then when it’s a 203mm lens and when it’s a 356mm lens? How do I properly figure out what “0” is considering there are multiple combinations and by design the iris, when wide open, doesn’t fully expand to the inner surface of the tube?

242351

[MadJake]

What you have a casket set or 'vade mecum'. These sets of lenses gave a large combination of focal lengths. They were generally quite slow. The longer the focal length the slower the lens. You might find more info by searching for 'lens casket set'. A lot of them were very similar. I had a set years ago which I bought for wetplate but they were a bit too slow.

[Nitesail]

Thank you, that helps. Slow indeed! That means that, doing the math, at “152mm” wide open would be about f/8 and at “356mm” wide open would be about f/22. However, since I won’t be using it for portraits, and since the shutter is the lens cap, I might find a use for its slow nature.

[Jim Andrada]

I have a small Protar casket set and I love it. I think the images are extremely nice to look at. Slow - so's making pasta from scratch which we do all the time. It tastes better.

[Nitesail]

I have a small Protar casket set and I love it. I think the images are extremely nice to look at. Slow - so's making pasta from scratch which we do all the time. It tastes better.

The test shots I’ve done definitely have a unique look to them. Wide open everything has a soft glow to it, over the entire scene. Once stopped down it looks a lot like other old lenses I have. I think this will pair well with my 5x7 Korona and contact printing.

[Steven Tribe]

There should be another table for this casket set which gives a “translation” of what “0, 1……5” means in terms of F value for the various focal length combinations possible. For many of the combinations, some of the F stops will be well past the defusion limit. It is not difficult to relate the EFLs given in the existing table with the actual pupil diameter to get the effective F. They will turn out to be odd number series - not 8, 11, 16 and so on!

[jnantz]

sometimes with convertible lenses the stop change is about 1.25 with each interval.
I have a casket set and there are 5 focal lengths and the relative stops go f 5.5, f7,f 9, f11.7, f37
https://en.wikipedia.org/wiki/F-number
has a chart that has more than just the standard 2/4/5.6 &c. which might or might not be of help.

good luck!

[Nitesail]

Thank you for the replies, much appreciated!