View Full Version : 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.

22-May-2019, 06:14
[Edit: forgot to insert the link]

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.

22-May-2019, 06:50
[Edit: forgot to insert the link]

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

22-May-2019, 07:07
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.

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!

22-May-2019, 12:50
“I would think that this issue of equivilant aperature determination had come up before.“

It has. It is a rather well-understood situation. Have you checked out the comparison table?. It’s old but. For me at least, accurate enough.

Just because it’s old is no reason to think that there can’t be trust. :).

But I’m with you on “trust but verify”. I verify by looking at negatives (or Polaroids back in the olden days) rather than calculating and extensive testing.

Drew Bedo
22-May-2019, 12:53
Steven: Sounds good. But I am confused about x/y . . .which is which?

If: GG measurement of the known focal length is...………………….M1
` The marked focal length of the known lens in mm is...………...f1
GG measurement of the unknown focal length is...………………..M2
The actual focal length of the unknown lens in mm is …………...f2

Then the equation would be............................................……...f2=f1(M1/M2)

Do I have that right?

Drat! I cannot get everything yto line up nicely.

Steven Tribe
23-May-2019, 00:15

You talk about GG, but it is far easier to compare known and unknown focal lengths by keeping the lenses away from flanges, lens boards and cameras!

Just go to an interior wall ( white or light wood) - in a place that will not attact attention to other members of the household! Point lens #1 towards a well lit window. Adjust distance to wall to "engage" focus - make one pencil mark with one hand, and then another when the focus is stable again. It is fair easier to do this than with the height of the nearest tower/chimney because of the light/darkness transition around the edge of the window frame.

I suppose the dedicated nerd could mount a kind of "trophy board/kids growth through the years" board, recording previous measurements and removing the need for new focal length bench marks everytime an "unknown" lens arrives in the post!

Emmanuel BIGLER
23-May-2019, 08:36
Hello from France!

The method proposed by Steven Tribe is one of the best and simplest, provided that you know the real focal length of the reference lens, and will allow you with a simple ruler to determine the unknown focal length within an accuracy of a few percent.

However may I dare to propose another method, probably less precise, but which does not require any other reference lens. And of course, like Steven's method, which does not require to know where the principal or nodal points of the lens are located. You only need a large format camera, a wooden stick, some gaffer-tape, a table, a piece of paper and a pencil ;)

The method is based on the fundamental relationship between the focal length "f" and the image displacement "y" (for a far-distant object) on the ground glass when you rotate the whole camera by an angle "theta".

The formula is : y = f tan(theta) when starting from an image centered on the optical axis of the lens, i.e. in the middle of the ground glass with all shifts zeroed.

Now assume that we rotate only with a small angle, less than 10 degrees, tan(theta) can be approximated by theta (in radians) or approx. theta/57 in degrees. And with small angles we do not need to measure from the center of the ground glass, we simply measure the whole displacement of the image across the ground glass, provided of course that during the rotation, our reference object (a distant spire, a distant electric pole etc) projects an image staying within the limits of the ground glass.

Find a wooden stick, about one meter (20") long. Firmly attach one end of the stick to the tripod head, just under the camera and above the panoramic movement, so that the stick will rotate exactly like the whole camera. Support the other end of the stick on a table, this requires to set the tripod to the proper height. When the camera rotates, the end of the stick will move on the table. Tape a piece of paper underneath and measure the displacement of the end of the stick for a given displacement of the image on the ground glass.

Let, for example, the image move by 50 mm (one inch) on tr ground glass.
If the unknown focal length is one meter, like the wooden stick, the end of the wooden stick will move by 50 mm when the image moves by 50 mm.
If the unknown focal length is 500 mm, the end of the wooden stick will move by 2x50 = 100 mm
If the unknown focal length is 250 mm, the end of the wooden stick will move by 4x50 = 200 mm

In general, for a displacement "d" at the end of a one meter long stick (20"), and for a displacement of the image of 50 mm (1") on the ground glass, the focal length of the unknown lens is simply
f (in mm) = 50x1000 / (d in mm).

For example, if the focal length is 300 mm, the displacement "d" will be 50 x 1000 / 300 = 167 mm.

I like very much this way of defining the focal length, because it is totally independent from the actual lens design, you do not have to care for the rear nodal or principal plane H', located one focal length ahead of the focal point. But on an antique brass lens, the location of H' is never engraved ;) and if it is a telephoto design, H' will probably float somewhere in air in front of the lens!

It is important that the reference object is located as far as possible from the camera, but if this condition is respected, you can rotate the camera around any rotation point, the displacement of the image will be the same for a given focal length.


This for finding the value of the focal length.

Now regarding the f-number "N", it is defined with reference to the diameter "a" of the entrance pupil, not the actual size of the iris.

N = f/a

You can measure the diameter "a" of the entrance pupil by illuminating the lens backward, with a small flash light located on the optical axis, at the focal point. Focus the lens at infinity with the ground glass, remove the ground glass and try fix the small flashlight at the center of the ground glass, without the ground glass.
In front of the lens you'll see a beam of light, intercept this beam with a piece of paper, placing it as close as you can from the first lens element. You'll see a parallel projection of the iris, its size is the size of the entrance pupil. This size can be significantly bigger than the physical diameter of the iris, but now you do not care for the physical iris, since you got the right measurement!
If the iris is not perfectly circular, estimate approximately the diameter, no precision is required here, you are allowed an error of about 10% on the measurement of the diameter of the entrance pupil !

Have fun!

David Lobato
23-May-2019, 13:19
Hey Drew, good luck with your lens. Keep us posted.