View Full Version : F-stop adjustment needed when swapping a shutter to a different f.l. lens?

John Schneider

29-Sep-2008, 09:15

I have a Copal 1 that S.K. Grimes engraved for a 240 Germinar, and I'd like to mount it in my 300 Germinar instead. I'm sure I could find the exact adjustment needed if I waded through the math, but does someone know offhand approximately how much I'd need to adjust the marked f-stops for the 300 Germinar?

Jon Shiu

29-Sep-2008, 09:23

Hi, 300/240 = 1.25, so, multiply your marked f stop by 1.25.

For example marked F8 x 1.25 = f10. Or, a little less than 1 stop difference.

Jon

John Schneider

29-Sep-2008, 09:33

Thanks. I considered that it might just be the ratio of the focal lengths, but then figured that was too simplistic to be correct.

Oren Grad

29-Sep-2008, 10:10

Not sure it would be strictly proportional if you were moving between two different lens designs that had different effects on the entrance pupil - maybe one of our optical experts can help here.

Peter K

29-Sep-2008, 11:21

It's explained here by Emmanuel Biglerhttp://www.largeformatphotography.info/forum/showthread.php?t=32332&highlight=f-stop

Nathan Potter

29-Sep-2008, 12:26

As Bigler describes above there are other corrections, (perhaps small) that depend on the entrance and exit pupils of different lens designs. However the two Germinars may be of sufficiently similar designs for the Shiu scheme to work accurately enough. As described in a recent post I use the GG with spot meter approach to do a comparison to a lens of known aperture. But look for my error cautions at the end of the post.

Nate Potter, Austin TX.

Emmanuel BIGLER

30-Sep-2008, 00:32

However the two Germinars may be of sufficiently similar designs for the Shiu scheme to work accurately enough

Yes, certainly, if the two designs are very close, every dimension is supposed to be scaled proportionally to the focal length from one lens to another in the same family.

So in order to keep the same f-number, the physical iris diameter should be scaled by the ratio of the focal lengths. If the physical iris diameter is not scaled like when swapping shutters, conversely the actual f-number changes proportionally to the focal length as described.

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I realize that in the long post quoted above, I'm using the term "numerical aperture" incorrectly, it should be : f-number N.

"numerical aperture" (N.A.) has a different meaning in optical engineering; e.g. for microscope lenses the numerical aperture is engraved on the lens, like N.A. = 0.40 and is equal to N.A. = sin (alpha) where alpha is the maximum angle of rays joining the centre of the field to the edge of the entrance pupil.

Dan Fromm

30-Sep-2008, 01:52

Emmanuel, IIRC Numerical Aperture is related to relative aperture (f/ number) by:

N.A. = 1/2*ra

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