Determining aperture of antique brass lens?

**Drew Bedo** · 22-May-2019, 05:24

I have gotten a vintage brass lens. Not a genuine antique with a famous name and waterhouse stops. It is unsignerd or unbranded and has a modern-ish looking rting set diaphragm with aperture numbers that do not correspond to modern f-stop numbers.

The focal length seems to be about 300mm. Getting it mounted on a lens board just now and when it comes back it will go on my 8x10 to see what coverage it has. Howe is the actual the focal length best measured?

Getting a more accurate focal length will help in determining the f-stops.

How may I determine the aperture wide open . . .and how may this be done for the other diaphragm settings? I'd like to be able to create a conversion chart relating the lens' numbers to modern f-stop numbers.

**BrianShaw** · 22-May-2019, 06:14

[Edit: forgot to insert the link]
https://en.wikipedia.org/wiki/F-number

Look in the “History “ section. There is a comparative table. Is your lens in one of these? All of my old brass lenses are US, in which US16 is f/16... it’s easy to figure out from there.

Focal length... I focus infinity on ground glass then measure btween the standards.

**Dan Fromm** · 22-May-2019, 06:18

Drew, as Brian said there are other aperture scales. Tell us the numbers engraved on your lens.

**Two23** · 22-May-2019, 06:50

Originally Posted by BrianShaw

[Edit: forgot to insert the link]
https://en.wikipedia.org/wiki/F-number

Look in the “History “ section. There is a comparative table. Is your lens in one of these? All of my old brass lenses are US, in which US8 is f/8... it’s easy to figure out from there.

.

No, I think it's f16 = f16 for the US (uniform system.)

Kent in SD

**BrianShaw** · 22-May-2019, 07:07

Originally Posted by Two23

No, I think it's f16 = f16 for the US (uniform system.)

Kent in SD

Yes, Kent... you are correct. I’m still drinking cup #1 of coffee and apparently my performance is diminished... or senility has set in. Could be both. I’ll fix my post so as to not further confuse the conversation. Thanks!

**Drew Bedo** · 22-May-2019, 09:02

I do not now have the lens in-hand. It is at Professional Camera Repaie here in Houston, getting mounted to a board (the board is being modified to acdept the lens). When I get it back next week I will reopen this question.

The lens thread is American (ANSI?) as a metric ring that sort of fits does tighten, but is cross threaded.

I'll try to post pictures. It looks great on the shelf. The iris blades are yellow brass. Works a bit stiff, but smoothly. Next step is a Packard Ideal shutter big enough.

**Drew Bedo** · 22-May-2019, 09:10

Brian: Than ks for the link. I will check it out when the lens comes back from Professional Camera Repair.

Regarding focal length: Sure, that is how I deal with correction for bellows extension with "modern" lenses in the field. However, this vintage lens mounts at the rear of the lens barrel, so the optical center ("Prime Node?) of the lens will be well forward of the front standard. As I will be trying to verify or figureout the focal ratio(s), the f-stops, I was thinking to be more formal in working this out. Lacking specific knowledge, my first approcimation would be to measure from the front of the geround glass to the iris blades.

However, I welcome any suggestions.

**BrianShaw** · 22-May-2019, 09:30

Measure and calculate both ways, Drew. I think you’ll find the difference is in the noise do all practical purposes.

Is there a reason you can’t trust the original markings, once they are correct with f/stop?

**Drew Bedo** · 22-May-2019, 11:02

Mar5kings on the lens' aperture ring do not correspond with the current f-stop progression. I do not remember exactly what they are just now. The lens is being mounted by a technician. It will come back next week. The numbers don't match the readout on my light meter, so I am looking at creating a chart of equivilant f-wstops for it.

The trust issue also comes from the age. I am guessing that this comes from the early 20trh Century . . .before 1930 maybe. exposure conventions were noty standard across the board, so I want to verify what-is-what.

The current popularity of alternate processes has created a demand for these lenses compared with thirty years ago when they couldn't be given away at camera shows. I would think that this issue of equivilant aperature determination had come up before.

**Steven Tribe** · 22-May-2019, 11:04

I always do the focal length measurement of an unknown lens by doing a comparison with the image cast by a lens of known focal length. Get a window frame in focus on a white wall. Make two marks that indicate the width or depth of the illuminated glass. Measure the distance. Now repeat with the known lens. Two new marks showing the illuminated window.
Measure the new distance. Now it is a question of multiplying the known focal length by x/y. Of course, you choose a fixed measurement which almost fills up the frame to increase precision!