Re: IR Focusing with Canon EOS lenses

[Otto Giesenfeld <otto.giesenfeld@xxxxxxxxxxxx>]

--On den 21 juli 1999, 19:32 -0400 "Mark W. Johnson"
<markwjohnson@xxxxxxxxxxxxx> wrote:

> I think if you have a specific couple of fixed lenses or a particular
> zoom lens that you want to use with IR, it is worth doing some carefully
> composed test shots with distinct objects at measured close, mid-range,
> and near infinity distances, then to shoot the same scene at several
> well annotated lens focus settings with the lens wide open.  After
> developing the pictures and comparing them with your notes, you should
> be able to come up with some pretty good guidelines for using your own
> equipment, and when you stop-down the lens, you'll get the assistance of
> depth of field.
> 

Just a short addition to this: It would be a good idea to put a ruler with
a clearly visible distance scale in the middle of the test pictures (except
for near-infinity distances...) and to write down for each picture which
part of the ruler is in focus for visible light. (The ruler would have to
be in something like a 45-degree angle from the optical axis, of course.)
This way, the steps between the different focus settings tried could be
larger, as it would be possible to interpolate to get the exact correction
needed. Instead, the number of distance ranges or focal length settings
could be increased. 

Actually, thinking more about it, I'll make this note a little longer. All
you would need for a certain distance range with a certain lens/IR
filter/zoom setting is to take two pictures of the ruler. Do as follows.
(If anyone sees a problem with the procedure, please correct me. I have not
tried it myself yet, but I will as soon as I have time to.)

1. Take one picture of the ruler, setting visible focus exactly at the
middle of its scale.

2. Turn the focus ring of the lens a certain amount. Write down how much
you turned it. (In millimeters, degrees, fractions of inches or whatever.)
Call this number A.

3. Take another picture of the ruler with the new distance setting. (No
need to write down where visible focus falls.)

4. When the film is developed, check how many steps of the ruler scale
focus moved between the two pictures. Call this number B. Also check how
many steps it would have needed to move in order to bring focus to the
middle of the scale. Call this number C.

5. The necessary focus correction can now be calculated as A*C/B.

Obviously, approximaitons are made, but as long as focus shifts are
relatively small, the results obtained with this metod ought to be good for
all practical purposes.

Otto Giesenfeld
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