Bellows Extension/Focusing Distance Formula

[Jerry Bodine]

Short working distance. Possibly, great stress possibly, so-so image quality.

Dan,
Would the "so-so image quality" be due more to the lens design or to the focal length?
Jerry

[Dan Fromm]

design

[mircea nicolae]

Hi guys,

I am looking at a similar problem as the one in this thread, but from a different point of view. My questions sounds like this.

If D is the distance between the lens and the subject (focused), F is the focal length of the lens, and E is the distance between the lens and the ground glass, is there a mathematical formula that calculates E as a function of D and F?


I am trying to figure out the lens to flange distance for a 135 mm apo sironar s lens placed at 12 meters from the subject, when focused.


Thanks.

[Dan Fromm]

mircea, I find all this easiest to think of in terms of magnification.

The magic formulas are, in your notation:

D = F*(m + 1)/m where D = front node to subject distance, F = focal length and m = magnification

whence m = F/(D - F)

E = F * (m + 1) where E = rear node to subject distance

whence E = F + (F^2/(D - F))

For most of the lenses used on LF cameras, the nodes, front and rear, are approximately at the diaphragm. This is far from true for telephoto lenses.

The rear node to flange distance for lenses is very variable, depends on the lens' mechanical design. Measure!

[mircea nicolae]

Thanks a lot