Aperture calculations...?

[Randy]

I am sure this has come up before but I can't seem to find what I need to know. I calculated waterhouse stops for an old brass lens a couple years ago with the help of Galli, but that was then, this is now :( and I forgot...and I suck at cipher'n.

I have an uncoated 18" f/10 Wollensak Process Velostigmat.
I unscrewed the front cell and taped a 12" coated meniscus to the front. Infinity focus is now 12" with the rear cell still in place.

I want to calculate the aperture at wide open, all the way up to f/64 on the existing aperture dial.

Here are my measurements:

FL - 12" (300mm)
Front element diameter - 65mm
Aperture openings:
f/10 - 44mm
f/16 - 29mm
f/22 - 20mm
f/32 - 12mm
f/45 - 9mm
f/64 - 7mm

I guess what I need to figure out is what the aperture actually is when set wide open (f/10 on the dial), and what each succeeding f/stop actually is. I am guessing f/10 on the dial is probably more like f/8, f/16 on the dial is probably closer to f/11, and so on.

If someone could give me some instructions to at least get me started, or offer a web page that might give instructions, I would be grateful. I am anxious to give this lens concoction a try on 5X7.

[vitality]

Look how easy is that:
f = focal length
You state your aperture as f/10 - so diameter of aperture will be focal length (f) divided by 10. In your original state FL was 18" (450) divide it by 10 - you get yours 45mm of aperture diameter (+\-).

To make opposite calculation, you simply need to divide FL by diameter of aperture. Eg. new FL = 300mm, so 300/44 = 6.8 and etc...

[Randy]

vitality, so it look like the existing stops will be about a stop off - f/10 on the dial will be f/7, f/16 will be f/11, f/22 will be f/16 and so on. Does that sound right?

[vitality]

vitality, so it look like the existing stops will be about a stop off - f/10 on the dial will be f/7, f/16 will be f/11, f/22 will be f/16 and so on. Does that sound right?

Yes, that's right.
ex f/16 = 300/29mm = 10.3 (or approx f/11)
ex f/22 = 300/20mm = 15 (or approx f/16)
...

[Randy]

Thanks a bunch. Now time to do some experimenting.