How can I estimate the f-stop equivalents for the numbered apertures on a No.1 Pocket Kodak?
Asked 5/18/2019
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I have a No.1 Pocket Kodak with the basic "single lens" shutter plate, and the aperture settings are marked only as 1, 2, 3, and 4 rather than f-numbers. I’d like to use modern 120 film and estimate exposure more reliably, possibly by comparing with a metered 35mm camera.
Is there a practical way to determine the f-number equivalents of these numbered stops? The aperture appears to be in front of the shutter and lens, and I can roughly measure the opening diameter. I also have the original exposure table, but it was intended for much slower film of the period, so I’m not sure how directly it applies today.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
7y ago
2 Answers
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Well, you can likely measure the focal length (from lens center to film, assuming ∞ focus). The entrance pupil diameter, being outside of the lens as opposed to a modern camera, is really easy to measure (you actually give the numbers). Your stop numbers will be the focal length divided by the respective entrance pupil diameter (assuming all of the aperture is effective even at the largest stop). The location of the aperture is not perfect so there will be a bit of vignetting, but the math for a single-lens camera is pretty much what you are working with even with modern lenses: all the additional elements are for making the lens better match the single-lens theory.
Originally by user84875. Source · Licensed CC BY-SA 4.0
user84875
7y ago
0
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Yes. The practical way is to estimate the f-numbers from the lens geometry:
f-number = focal length ÷ entrance pupil diameter
On a simple folding camera like this, the entrance pupil is easy to measure because the aperture is in front of the lens. If you set the camera to infinity focus, measure the focal length (roughly lens center to film plane), then measure the aperture diameter at each setting. Divide focal length by aperture diameter for each numbered stop.
That will give you approximate f-stop equivalents for 1, 2, 3, and 4. Because the aperture placement is not ideal, the result may not be perfect and there may be some vignetting, but it should be close enough for exposure work.
A second rough check is the old exposure table: based on period film speeds and Sunny 16 reasoning, the widest setting may be around f/16 or in that neighborhood, but measuring is the more reliable method.
So the best answer is: measure focal length, measure each opening, calculate f/N, then test a roll and fine-tune from there.
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