Modern f/stops

[FM2N]

Hello All,
I have a Brownie ballbearing lens with the following openings-
US8-near portriat
16-average view
32-distant view
64-marine, clouds, snow
and a mark just past that i believe is 128.
COuld you tell me how to figure out what the equivilent modernf/stops would be?
The focal length at infinity is 7 inches.
I measured as well as I could the openings at each setting and they are as follow
8=14mm
16=12mm
32=10mm
64=8mm
128=6mm
Thanks for your help
Arthur, NYC

 

[keithwms]

The f/# should be ~7*25.4/aperture diameter, right? So the "8" setting would be f/13 or so, the next one f/15 or so, and the "128" setting would be f/30 or so. I'd be inclined to check this with instant film.

 

[Ole]

US stops are simple:

US 16 = F/16

And US 8 = f/11, US 32 = f/22, and so on.

 

[Christopher Walrath]

I'm looking at these diameters and numbers and the size reduction is not a geometric series (base 1.414). The 8 is around f/13 and the 16 is f/15. But as we continue the 32 is f/18, 64 is f/23 and the 128 would be f/28. Not a typical aperture series, but if your diameter measurements are accurate this is the series. Definitely try some test prints first to be sure.

 

[Larry Bullis]

There seems to be some contradiction between the calculations and Ole's post, which is the explanation that my understanding supports. These f/13, f/15, f/30's etc. don't make sense. The US (not United States, you Nacirema, but Uniform System) preceded the f/stop system we use today, phased out around 1909. I have 2 Goerz Dagors marked in these stops, and the way Ole has expressed it has worked for me perfectly in making 8x10 negatives since 1970. Since the Dagor was first released in I think 08, I know these lenses to be among the first of their type. Using the cross at f/16 has not once resulted in an error.

So there has to be some discrepancy, somewhere, either in the measurements, or they simply can't be US numbers. Which is it?

 

[keithwms]

Arthur, what is your degree of confidence in your measurement of the aperture diameters, and when you measured these, if the system has blades as opposed to drop-in aperture rings (?!) did you measure the max or min diameter between the blades... or an average? And is the 7" focal length known for sure?

 

[phfitz]

Arthur,

Ole would be correct, Kodak would have used the US scale or the f/stop scale.
US 8 = f/11, US 16 = f/16, US 32 = f/22, US 64 = f/32, US 128 = f/45
measurements would be optical, not physical AND might be from the rear looking forward thru the lens.

Have fun with it.

 

[FM2N]

My measurements are as close as I can get with my ruler. The focal length is 7 inches. I measured by putting the pointer at the appropriate stop and measured across the opening. As soon as the camera is up and running I will try Ole's and Phfitz's f/stops and let you all know how it worked out.
Arthur

 

[Whiteymorange]

This has been helpful to me in the past.

 

[FM2N]

Thanks for all the help...lastly could you help me find a bulb release for this lens?
Arthur,NYC

 

[Christopher Walrath]

Whitey, that was some great referential info there. I'm curious, why would US1 equivocate f/4? Was that an old aperture extent or something?

 

[Whiteymorange]

Guessing here. You keep cutting the US in half as you go down, US16 = f/16; US8 = f/11; US4 = f/8; US2 = f/5.6 and US1 would equal f/4. The system wasn't very flexible and seems, for the most part, to have kept to whole numbers.

 

[Ole]

At the time the US was introduced, the fastest lenses were f:3.5 Petzvals. So there was simply no need to be able to mark anything faster than f:4 - or US 1.

 

[GeorgesGiralt]

Hi !
I was told that in order to measure the exact opening of the diaphragm of a lens, ons should focus the camera at infinity, and then, without moving the focus, place a piece of B&W paper in the front cap and expose it from the rear (with a spotlight placed at the ground glass). the diameter of the black spot on the paper is an exact measure of the lens aperture as it should be optically.
I wonder if this method is good but I think it is making sense as it use a spotlight at one focal lenght from the lens, and measure the opening as projected...
Maybe one of you can comment and explain this to us ?

 

[phfitz]

GeorgesGiralt,

That should work well BUT some lenses will give truely strange measurements that way. Some lenses are measure in the reverse direction or they do not match the aperture scale. Optical illusion of the lens design. Try it front and rear to see which way matches wide open.

Have fun with the hunt.