As a guide for figuring additional extension required and working distance
. . .
*Additional* extension equal to the focal length of the lens is required
for 1:1 magnification . . . which is life-size on the film . . . and which
must be something 24x36mm or smaller in size to fit on the film. That's
with the lens itself set to infinity focus. Working distance at 1:1
magnification is twice the focal length in front of the front lens node . .
. which is typically somewhere inside the lens . . . it's not the physical
front of the lens. For less magnification, less extension is required and
the working distance increases.
Other benchmarks to estimate how much extension might be required:
1:2 magnification (1/2 life size on film):
1/2 focal length added extension; working distance of 3x focal length from
front lens node.
1:4 magnfication (1/4 life-size on film):
1/4 focal length added extension; working distance of 5x focal length from
front lens node.
If you want to know how far the front lens node is behind the physical lens
front, you can estimate its location at about 1/4 to 1/2 the physical
length of the lens (not its focal length). Exactly where the front lens
node is can vary greatly depending on lens design. The only way to know
with precision is to put that specific lens on an optical bench in a lab
and measure it . . . or closely approximate it by performing an experiment
at 1:1 magnification (use a ruler as a target; 35mm film frame is about
1-1/2 x 1 inch) and measure back 2x focal length from the target ruler.
The lens helical on a fixed focal length lens can add a little extension if
focused closer than infinity. Indeed, for lenses that are *not* "internal
focus" which are very nearly all fixed focal length lenses . . . the
focusing helicals are fairly short (compared to focal length), varible
length extension tubes. Nearly all zoom lenses are internal focusing;
haven't seen one yet that isn't, but it doesn't mean someone, somewhere, at
some time hasn't made an external helical focusing zoom.
-- John Lind
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