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View Full Version : Trying to label unmarked stops on Darlot Lens

Terence Falk
27-Mar-2017, 14:02
hello,

I have a very old Darlot Paris Single View lens that has a rotating disc of stops but no markings on them at all. How could I determine what f/stops these holes represent?

thanks

Fr. Mark
27-Mar-2017, 21:41
If you can measure them and if you know the focal length of the lens, you can work it out mathematically. Even approximate values would be helpful, I imagine.

bizarrius
28-Mar-2017, 00:31
i guess... http://www.punitsinha.com/resource/aperture_focal_length.html ?

Two23
28-Mar-2017, 06:23
I might be wrong, but I would start by going on the assumption that each hole allows half as much light as the next.

Kent in SD

Steven Tribe
28-Mar-2017, 07:20
Find your size lens here on page 17!

http://www.antiquecameras.net/1890lenscatalogue.html

It will give the real focal length.

Devide this figure by the diameter of "holes" and you have the F-value for each. When there are only 4 or 5 hole, the apertures may not follow the usual sequence.

Terence Falk
28-Mar-2017, 14:08
thanks so much for the helpful ideas!

lungovw
27-Apr-2017, 19:40
Terence,

I am collecting old brass lenses and often face the same problem. It is always helpful to check literature, when you can find it. But I feel more reassured if I can combine or double check it with some experimental procedure. I usually take pictures of front element with different stops and then measure the entrance pupil (not the hole size) comparing sizes in pixels. This is just a procedure to measure something that is not accessible, that is an image. I describe the method here: https://apenasimagens.com/en/measuring-lens-aperture/. In case you don't know the focal length, I also prepared a routine that takes several sets of image size, subject size and lens trip (up to 12). Then a linear regression is made in order to show the focal length. I like that because it gives you an idea of consistency of data through a simple statistical algorithm. I have that here: https://apenasimagens.com/en/measure-find-lens-focal-length/. You have to be in the mood of doing this home lab work. I understand that the quick way is just about installing the lens in your camera, focus your subject and shoot.