Figuring Aperture Scale, Missing on Dagor

[goamules]

Hi,
I'm ready to try a tiny barrel f6.8 Dagor, but the aperture has no scale. Is there a way to get a close swag on making the hole different f sizes?

I shoot wetplate, so wide open at f6.8 will work. But I hear these Dagors are so sharp, thought I'd try it at some tighter apertures. I could just close it "halfway" and "mostway" and guess....

The lens was discussed here, with a pic.
http://www.largeformatphotography.info/forum/showthread.php?t=30984

Thanks,

Garrett

[Ash]

There is definitely an equation to work it out, since you know aperture and focal length.

I can't remember what it is, but it's to do with the apparent aperture (as viewed looking at the lens) when closed down as a fraction of the maximum and the focal length. No idea off the top of my head, but I'll get back to you if I can.

[Ralph Barker]

If you pre-calculate the standard f-stops for the focal length, you can then measure the apparent aperture looking through the front of the lens, and mark a scale made from tape or other material. You can use a rule marked in millimeters, or use a dial caliper, which is probably more accurate. Using only one eye helps, too.

For example, f/8 = 1/8 = 0.125 x 120mm = 15mm aperture opening

[Peter K]

Two weeks ago in another thread was an explanation to calculate the f-stops http://www.largeformatphotography.info/forum/showthread.php?t=32332
This method works with long focal lengths.

Peter K

[Ole Tjugen]

Adjust the aperture untill it's almost but not quite visible through the front of the lens. Then unscrew (at least) one of the lens cells. Measure the aperture opening. Set a mark on the aperture scale. Close the aperture until the opening is exactly half of what it was. Set another mark, that's two stops down from wide open (f:13.6).

Repeat as necessary.

[goamules]

These are all good, helpful plans. I thank you. I'm going to the shop now to make a lensboard for the speed graphic. I'll set up some aperatures, and try it tomorrow!