Calculating f-stops

[photosailor]

I got a Wollensak f4.5 241mm Raptar with an 8x10 view camera I bought a while ago. It wasn't mounted on a lens board and hadn't been used with the camera, just something thrown in with the deal. The Alphax shutter has three f-stop scales, but none matches the f/4.5 marked on the lens (one starts at f/7). I got to thinking that creating a scale for this lens couldn't be too difficult: just measure the diameter of an iris opening and divide by 241--bingo! the f-stop.

I measured 32mm when the scale read f/8; doing the math that is .1327 and f/8 is .125 so it seems that my simplistic formula is right. Except that f/8 was on the scale that starts at f/7.

So my question is: is the f4.5 Raptar maximum aperature f/7 when mounted in Alphax shutter and should I be using that scale? I haven't had the shutter CLA'd so I don't know how accurate the speeds are so I can't interpolate the f-stop from a correctly exposed negative.

Amazingly, this lens seems to cover nicely the 8x10 format.

Any observations you have about this lens will be appreciated. Thanks!

[aduncanson]

This is a frequent topic on this forum. There are several approaches proposed. You might want to search for the topic. To me, the following is the best (most comprehensive) answer given.

http://www.largeformatphotography.info/forum/showpost.php?p=309634&postcount=2

[Dan Fromm]

You don't want to calculate, you want to measure.

Pick up the shutter, unscrew the lens' rear cell. Open the aperture wide, hold the lens at nearly arm's length with the lens' front facing you, measure the entrance pupil's diameter. The entrance pupil is the aperture, magnified by the front cell. The calculate the aperture wide open. That's the most you can get with the Alphax. Now close the diaphragm slowly, until you can just see it through the front of the lens. Then open very slowly until it just disappears. The f/number the aperture indicator points to corresponds to the max aperture you can get with the Alphax. And there you are.

[photosailor]

Thanks for the suggestions!