Need help calculating f-stops of unmarked barrel lens

[medform-norm]

A while ago we picked up an old Ica Extra-Rapid Aplanat, f:8, 19,5 cm doorstop with amzingly clean glass. I believe this is a Rapid Rectilinear lens design, very likely to be sharp in the centre, soft everywhere else. We want to try it out, but the thing lacks f-stops. I looked around on the web, but it's a little confusing what gets said on the subject. It seems one can calculate f-stops by dividing focal length (195mm) by the diameter of the lens opening. That doesn't sound undoable. Question is: how and where does one measure this diameter? I can't very well put my measuring tool inside the lens, can I? What's our aim: this lens is not entering a math contest for the most precise instrument. All we want is at least some idea of exposure in order to test if it is going to continue its life as a paper weight or if it gets its own lensboard.

(Might there be a method involving the use of a well-calllibrated light meter like a Gossen Profisix? That would be handy.)

Thanks for lending your mind to this question.
Norm

 

[Nick Zentena]

You're supposed to measure the apparent openning not the actual opening. At least that's my understanding. Put the ruler/whatever on the front and measure what the opening looks to be. The focal length likely isn't exactly 195mm either so I'm not sure how much slop you'll end up with. But once you've got it figured out do some test shots and calibrate your film speed with the lens. Should be good enough.

The only thing I can think of the meter being used for would be after you've got an intial F/stop. The meter could then be used to calibrate the rest of them. I think that would work but you need a starting point. Then using the meter every time the meter shows a 1 stop change in light mark that. If that makes sense.

 

[Dave Parker]

It would seem to me that you have a +- intial f/stop based on the description you have provided, you have described it as a 195mm f/8 is this what is marked on the barrel? if so you should be able to fairly close figure the f/stops based on your intial information and stopping down and measuring the light out put with a light meter.

Dave

 

[Denis P.]

Norm, I was in a similar predicament a while ago, when I got some weird lenses to try out on my Speed Graphic. One of those was a hacked Helios lens for a 35mm format Russian Zenit SLR. When I removed its back element, I changed its focal length, and thus also f-stops (duh! - took me a wasted roll to realize why all shots were underexposed!).

Now, what I did is entirely unscientific, mind you... might not be "close enough" for your purpose...

I put a uniformly lit piece of white paper in front of the camera, and using a well-calibrated and reliable lens, with the shutter on "T" (open), measured all the f-stops using a good light meter (metering off the ground glass).
I wrote down the metered values, and replaced the lens with the one with "unknown" f-stops.

I metered again off the ground glass, and when I got the desired values (equal to the previously measured ones), I marked the f-stop positions with a pencil on the barrel of the lens.
After that I used a drawing program and made a DIY paper f-stop scale for the hacked lens, which I simply glued on...

Not too difficult, and probably not terribly precise, but the exposures are much closer to the standard now....

Denis

 

[medform-norm]

Ha, very useful comments and in line with what we were hoping for. So no very incredible difficult calculus that doesn't translate to real life practice, but basic thumb rules. We will try different methods, then we can compare.

And yes, it has marked on the barrel 19,5 cm f:8 so there is an initial f-stop to start with. Things aren't looking too bad for the Ica doorstop after all

Thanks, Norm