Determining F-Stop with convertible lens

[darter]

I have a 7" lens designed to be converted to 11". I have the f-stops for 7" but have no idea about the stops when it is at the 11" length. What is the best way to determine the 11" stops?

[clay harmon]

11 inch fstop appx = 7 inch stop times 11/7

This assumes that the 7 inch lens has entrance and exit pupils approximately the same size and the stop is in front of the single cell. This may not be exact, but is probably closer than your shutter speed is likely to be.

[C. D. Keth]

You can also place a pointy-source light on the film side of the lens far away enough to be effective infinity and measure the diameter of the projected beam coming out the front of the lens at the various marked stops. This way would let you make an aperture scale with standard stops for the 11 inch length or mark the scale you already ahev with an additional scale of standard stops.

[Helen Bach]

You can also place a pointy-source light on the film side of the lens far away enough to be effective infinity and measure the diameter of the projected beam coming out the front of the lens at the various marked stops...

Christopher,

Shouldn't the point light source be at the film plane (with the lens focussed on infinity) to project a beam with a diameter that equals that of the entrance pupil, or have I missed something? That's the normal way of projecting the entrance pupil. Less accurate, but you can just look at it and attempt to measure what you see.

Best,
Helen

[Ernest Purdum]

Hey, unless I'm missing something this is the easy situation. Since in a convertible lens you take off the front cell, you are now looking at a diaphragm without any glass in front of it so the actual diaphragm diameter can be divided directly into the focal length to get the f stop.

[Helen Bach]

Ernest,

Why would you prefer the physical diameter of the diaphragm instead of the diameter of the entrance pupil?

Clay's method looks the simplest, if the change in entrance pupil diameter is ignored.

Best,
Helen

[Ernest Purdum]

Helen, for some help in answering your question, I turned to the Focal Encyclopedia of Photography. In the entry "Diaphragm" there is a heading "Effective Stop Diameter" in which there are four paragraphs too long to quote entirely here, but is a good description of the situation. (The last paragraph includes an interesting method of measuring effective diameter.) The pertinent comment to your question, however, follows:

"If the diaphragm is in front of the lens, then the two figures are the same - i.e., the diameter of the incident beam of light is equal to the diameter of the aperture in the diaphragm."

Since Darter wants the f stops for the 11" configuration, in which the front cell is removed, the condition applies.

[Helen Bach]

Thanks Ernest, I didn't think about the front cell being absent - it's so obvious now! I'm so ditzy.

Best,
Helen

[C. D. Keth]

Christopher,

Shouldn't the point light source be at the film plane (with the lens focussed on infinity) to project a beam with a diameter that equals that of the entrance pupil, or have I missed something? That's the normal way of projecting the entrance pupil. Less accurate, but you can just look at it and attempt to measure what you see.

Best,
Helen

It should be, I explained very poorly in my haste. I actually just explained this same thing to someone asking how to figure out new stops for just the front cell of another lens. See that discussion at:

http://www.largeformatphotography.info/forum/showthread.php?t=20757

EDIT: I just saw that this is all for using the back cell of a convertible. Yeha, the physical diameter of the iris opening will be correct. Anyway, I'll leave the link for some light reading :P

[Ernest Purdum]

Christopher, the Focal Encyclopedia method I mentioned above is quite similar to yours except that they suggest exposing a piece of photo paper and taking the measurement from it.

[Ernest Purdum]

Helen, I think all of us occasionally miss the obvious. I know I have. Thanks for posting your "snaps".

[darter]

Actually, the rear element is mounted in front of the diaphragm. Funny, eh? But that is the only way it will work.