F stop not as indicated? - advice on f6-384 Rapid Aplanat No.3 Ser.D.F.8 needed

[oldparson.art]

Hi I need advice on Rapid Aplanat No.3 Ser.D.F.8 (R.O.J.A. vorm. Emil Busch, Rathenow) f 6-384
I had tested it on a large format (10x8) camera and used the widest aperture with no problem - but at the indicated f90 it way over-exposed the image (Paper negative). I tried a couple of other lenses at the same time successfully so thought I'd make some measurements of the diaphragm aperture with digital callipers and calculate the theoretical f-stop (dividing the focal length by the aperture diameter)https://www.flickr.com/gp/oldparson/ZVf1yH
As the F stop numbers increase the variance between the indicated f stop and the calculated one increases greatly.
Can you explain what is going on with this lens?
(I measured my three other lenses in the same way and had measured f stops that closely matched the indicated value.)
With thanks
Simon

[Corran]

I think your lens is using the old US scale for aperture. f/16 is the same in both our normal modern scale and the US scale, and then from there each doubling or halving of the aperture is one stop, as opposed to our progression. Yours is a bit more difficult due to being calibrated to whole stops from f/6 though!

Edit: or perhaps it is the Goerz scale below? Your measurements seem closer to the US scale though.

[Pere Casals]

measurements of the diaphragm aperture with digital callipers and calculate the theoretical f-stop

This is a wrong way to take the effective diafragm diameter to make the calculations. You have to use the diameter that is virtually seen from front of the lens, not the real hole in the iris. Think that depending on how the front cell compresses rays the same hole will clip more or less light.

Beyond that table above, you may place a DSLR in the back of the camera without the DSLR lens, projecting on the sensor instead on the GG. In that situation you may rate your effective apertures compared to a prime lens, by comparing dslr speed, with that equivalence you may easily nail exposures.

[Jim Noel]

I agree this appears to be an adaptation of the old US system. One wonders why they messed with it even more.

[oldparson.art]

Hi Corran,
Thank you for this info. - very helpful - will allow me to make proper use of the lens.

[oldparson.art]

Hi Pere, thank you for your correction of my measuring method - and I like the idea of using the DSLR here. I have used a DSLR to measure the exact speeds of Thornton Pickard shutters, so your suggestion is a nice variation of that - Cheers. Simon

[Bob Salomon]

This is a wrong way to take the effective diafragm diameter to make the calculations. You have to use the diameter that is virtually seen from front of the lens, not the real hole in the iris. Think that depending on how the front cell compresses rays the same hole will clip more or less light.

Beyond that table above, you may place a DSLR in the back of the camera without the DSLR lens, projecting on the sensor instead on the GG. In that situation you may rate your effective apertures compared to a prime lens, by comparing dslr speed, with that equivalence you may easily nail exposures.

Providing you take into consideration the increased distance that DSLR takes up in place of the gg.

[Corran]

Focused at infinity that makes no difference whatsoever.

[Pere Casals]

Providing you take into consideration the increased distance that DSLR takes up in place of the gg.

Of course... We use the DSLR viewfinder to check infinite focus, in that way we have the nominal lens to image plane distance.

With a monorail this is straight, we attach a macro extension ring to a lensboard we place in the rear, and that F-Mount (Nikon) hold the DSLR in place.