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# Thread: How to figure out F/stop scale on convertible lens when in different shutter?

1. Originally Posted by photomc
... My understanding is you take the focal length (say 210) divided by the iris opening (say 11.27 for f/16) which gave me 18.6 not 16. ...
No, you're supposed to take the focal length divided by the diameter of the entrance pupil of the lens. Only when there is no glass in front of the aperture will that be identical to the diameter of the iris opening.

Think of it like the glass in front of the aperture is a magnifying glass, so the entrance pupil is larger than the physical opening.

Placing a single lens ell behind the aperture gives better field flatness and less distortion. Try the same cell in both locations, you will be able to see the difference on the negative!

2. I believe I have a shutter aperture scale for a 210/370 convertible lens which is incorrect. Scale is too constricted and aperture should be smaller at each of the marked stops. As a picture paints a thousand words please see two attachments.

I would appreciate confirmation from the experts here that my conclusion is correct and that I will have to create a new, more expanded, scale for this lens and shutter. I am getting quite overexposed negs with this lens at the smaller marked apertures and assume it is mainly due to this scale problem?

Tony

3. That looks like someone has put a "linear" scale on a shutter with "non-linear" aperture. If the opening is 36mm at f:5.6, it should be 18mm at f:11, and 9mm at f:22.

That the aperture can stop down well beyond the last marked stop is not uncommon and doesn't have to be wrong.

4. Thanks Ole - My instincts and the arithmetic told me the scale was wrong but it's helpful to get a second opinion. Coming from 35mm and MF you don't watch the aperture blades move very often so this is another aspect to LF which takes you back to the fundamentals of exposure!

5. Ole - yes it's a non-linear aperture. See picture of new hand-written scale compared to old. Not pretty but effective enough for the time being. Not surprising I was getting 2x exposure at smaller apertures on the original scale.

Cheers, Tony

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