Magnification at maximum extension?

[David Low]

I have a half plate camera (roughly 5 x 7 film size), and when it arrived it had a front extension that I finally got round to fitting on tonight. This now gives a total length of 600mm from lens to ground glass. This is with a 203mm lens. I would be grateful if someone could tell me the formula for working out the maximum magnification and the distances needed for intermediate ones. Thank you.

[Dan Fromm]

m = ((film plane-to-lens' rear node distance)/(focal length)) - 1 = (extension/f) -1

With a 203 and roughly 600 mm of extension (board to film distance) you can go to roughly 2:1.

rearrange to get extension = f * (m + 1)

[David Low]

Thanks - that is easy to understand and very useful.