# Thread: Math: Calculating distance scale based on focal length, FFD, helical travel

1. ## Re: Math: Calculating distance scale based on focal length, FFD, helical travel

I put in enough significant digits to get me close to the distance I wanted. I also didn't want to compound rounding errors, though I imagine Excel keeps all of the decimal places. On my chart there is one more column with the degrees of turn so I can mark the focus scale on the Mercury and I wanted that as accurate as possible.

Regarding the 'true' focal length, yes I am aware that is dependent on the lens. Since I can't know the exact focal length of others' lenses, I used the default value from Schneider. If one wants to be exact they can measure their lens and edit the values accordingly. Or instead, I guess I can just not publish the rough values at all here? Sheesh.

2. ## Re: Math: Calculating distance scale based on focal length, FFD, helical travel

I wonder what the subject to be photographed is?
How is the distance from the lens focal point to the subject useful without knowing the depth of "satisfactory" [YMMV] sharpness, unless of course one is making maps/engravings/printed circuits?
With 3D subjects, that counts. And near-far varies with f stop. So why would .01 mm matter for practical , 3D photography??

3. ## Re: Math: Calculating distance scale based on focal length, FFD, helical travel

Good Lord, take 'em or leave 'em. The extension for the helical to reach 360 degrees was 3.75mm, not 3.7 or 3.8 so I kept it that precise, plus one more digit in some cases to get more accurate on the distance. Next time I'll just keep the data to myself.

4. ## Re: Math: Calculating distance scale based on focal length, FFD, helical travel

Originally Posted by Pfsor
Indeed, if you don't know the EFL of your lens and you don't know how accurate the infinity position of your lens is (down to 0.01mm both values) it is entirely futile to produce spreadsheets with values down to 0.01mm precision.
Calibration of the infinity position of the lens can be done using a groundglass on the film rails and a distant subject or diy collimation setup, and the EFL is known fairly accurately from the manufacturer's data. The point of this exercise is to get accurate enough extension values of the helical relative to the infinity position. Given that these generally wide angle lenses and focusing will be done by scale with a cushion of depth of field, it's true that sub-millimeter (or even millimeter) accuracy for film-to-subject is not necessary, but those are just numbers on a spreadsheet - who cares how many decimal places there are? Personally, though, I would start off with nice round numbers for the film-to-subject distances.

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