After writing the following text, I think the back of my
lenses will clear the shutter assembly of my TL120 by .02"
(.51mm). For the path which led me to these numbers, read on.


I performed my focus tests by mounting my lenses on a 4x5
view camera. I then focused the camera at 23m and measured
the distance from the front of the lens board to the glass.
(I chose 23m because there happened to be a street sign that
distance away from my tripod and I was unable to measure a
difference in the focus required for 23m and 900m.)

I took the same measurement when focused at 2.8m. (I chose
2.8m as it was very close the hyperfocal distance for f/22
and there happened to be a surface that far away on which to
hang my test chart.)

I was, of course, unable to directly measure the flange to
film distance on the 4x5. I had to add and subtract some
camera thicknesses but what I found was the flange to film
distance was between 3.12" and 3.14" for 23m and 3.17" for
2.8m.

Then I removed the TL120 lenses from its lens board, locked
the shutter open and measured the distance from the front of
the stock lens board to the film plane. To get the distance
from the mounting studs to the film plane, I had to subtract
the thickness of the lens board (.071"). 2.63" is what I
determined to be the distance from the front of the mounting
studs (back of the lens board) to the film plane.

So if we take 2.63" from 3.12" we get .49" as the distance
from the front of the new lens board to the back of the lens
board. This is how thick the lens board will be to focus the
lenses at 23m (which my measurements say is ~= infinity).

We can add .11" for the depth of the mounting studs to get
.60" between the front of the new lens board and the front of
the shutter assembly. This is how much space we have between
the lens flange and the shutter assembly.

The distance on the lens from the mounting flange and the
back of the lens element is .63". So my math says the lenses
will be .03" too long to permit infinity focus :(

But, as was discussed here a few weeks ago, I don't see a
need to focus at infinity :) If I focus the lenses at 2.8m,
the lens moves .05" farther from the film plane and I'm back
in business with .02" to spare!

So I _think_ it will work without having to shave the back of
my lens. Peter Thomson had to mill the back of his lenses,
though. There are two ways to account for this:
A) It may be that Peter has focused his lenses farther out
than I plan to.
B) I've messed up my measurements or my arithmetic.

If you have read this fay, your thoughts and analysis would
be most welcome.
________________________________________
John Thurston
Juneau, Alaska
http://stereo.thurstons.us