Helical focusing large format lenses

radiant · Jan 31, 2021

I'm a bit lost how to design helical focusing adapters for large format lenses.

For example on my 5x7" with 210mm lens the focal length needs to be adjusted for multiple centimeters if I want to focus from 1-2 meters to infinity.

And on my 6x17 with 90mm lens I've noticed that the focusing distance is really really small, maybe maximum 1 cm.

How these distances are calculated? I know it is related to lens design but is there some kind of ground rule / calculation formulas?

ic-racer · Jan 31, 2021

You can approximate with the thin lens formula: 1/p + 1/q = 1/f

Otherwise make a linear helicoid and calibrate in with empiric measurements (a ruler or tape measure).

radiant · Jan 31, 2021

ic-racer said:
You can approximate with the thin lens formula: 1/p + 1/q = 1/f

Otherwise make a linear helicoid and calibrate in with empiric measurements (a ruler or tape measure).

Thanks!

--removed wrong calculations--

Dan Fromm · Jan 31, 2021

OP, the formula given in post #2 above is correct. Arthur Cox' book Photographic Optics (available from used booksellers on line, not as inexpensive as I'd like) gives a fuller explanation.

radiant · Jan 31, 2021

New calculations:

So 90mm lens would need approximately 9mm of helical movement and 210mm would need 55mm movement.

reddesert · Jan 31, 2021

It is also useful to think in terms of the magnification, m. m = d_image / d_object. You can then rewrite the thin lens equation as:
1/f = 1/d_obj * 1/(m * d_obj)
and also find that, d_image = f * (1+m),
and the extension past infinity is e = m * f.
With a little algebra, the thin lens equation then gives: d_obj = f * (f/e + 1).

That means that, when you are at non-macro distances and f/e is largish, the extension required to focus at a given lens to object distance goes up roughly as the square of the focal length. This is why long focal length lenses need a lot of bellows (and also why they have shallow depth of field in the image space, but that's another story).

ic-racer · Feb 1, 2021

Also, realize that the folks at Linhof have all the equations, but their focus cams are all matched to each lens with bench testing (supposedly). So, no problem if the math is not perfect, you will still have to use a ground glass and tape measure to finalize your scale.

radiant · Feb 1, 2021

ic-racer said:
Also, realize that the folks at Linhof have all the equations, but their focus cams are all matched to each lens with bench testing (supposedly). So, no problem if the math is not perfect, you will still have to use a ground glass and tape measure to finalize your scale.

I did that for my 6x17 of course. But when you don't have any "body" or support it is pretty cumbersome to find out the needed distance. It is easy to say when you can move the lens for 10cm that you only need 1cm

Helical focusing large format lenses

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