Re: Infrared Focusing-The true Story?

["joe b." <joe.b@xxxxxxxxxxxxxxxxxxxxx>]

In message <9607262034.AD08846@xxxxxxxxxx>, Willem-Jan Markerink
<w.j.markerink@xxxxx> writes
>Some weeks ago, Joe Berenbaum pointed me to the stunning fact that 
>Rollei IR filters had this diopter build in, mentioned in some Rollei 
>literature. After buying a cheap Rollei filter, I can confirm that 
>the included filter manual (cute, never saw a filter manual before!) 
>also mentions this IR focus adjustment.
>However, I cannot see any curve in this filter; one of my fellow 
>filter-nuts tried it by reflecting the sun on a far away 
>wall....neither side showed any enlargment of the reflected 
>spot....nada niente effect. Are there any other ways to proof
>that a glass element is not plan parallel?

Although that book did say, and I quote; "the current IR filter provided
for the Rollei incorporates a very weak lens element" so presumably the
earlier Rollei(flex) IR filters didn't have this characteristic; that
was published in the 1970 edition of The Rollei Way. So;

a] you might have an earlier filter, or 
b] when it says " a very weak lens element" they might mean *very* weak! 

You could try comparing (measuring?) the size of the reflected image of
the sun with that from a known flat filter.


                          ------+------


There is an interesting passage in Gunter Osterloh's book "Applied Leica
Photography" in which the infrared focusing method is described,
although I wonder how many IR phptgraphers will bother with this method-
I have only used monochrome IR for landscapes with wideangles and
middling apertures and have not had focus problems, and after reading
this I did develop a slight headache... enjoy;

"Since lenses for conventional photography are not corrected for
infrared radiation, their focus setting has to be changed. The infrared
index, such as the red dot and the letter "R" that were engraved on
earlier Leitz lenses, can only be used under specific conditions,
because that index is valid for only one particular combination of film
and filter, and only for a subject at infinity. Therefore, Leitz no
longer uses such markings.

An additional lens extension of 1/200 to 1/400 of the focal length of
the lens can be used as a guide. A test is necessary if the optimum lens
setting is to be determined. To find that setting, the following
procedure is used. First, the best infrared setting for infinity is
established. Based on the premise that a lens extension of 1/300 of its
focal length represents an average value, the lens is not set at
infinity, but at a slightly closer distance, which should be the
equivalent of 300 times the focal length. 

With a 50mm (2") lens, that distance would be 
50 mm x 300 = 15,000 mm = 15 m (2" x 300 = 600" = 50 feet).

With a 90mm (3 1/2") lens, it would be
90 mm x 300 = 27,000 mm = 27 m (3 1/2" x 300 = 1,050" = 87 feet).

With a 135mm (5 1/4") lens, it would be 
135 mm x 300 = 40,500 mm = 40.5 m (5 1/4" x 300 = 1,575" = 131 feet) 

and so forth. Now fasten a strip of graph paper over the distance scale
on the focusing mount of the lens, so that the distance setting can be
changed by small increments of 1mm or 1/16" both to the left and to the
right. The aperture of the lens also affects focusing, therefore all
photographs should be made with the same aperture setting. The resolving
power of the lens in the infrared range is reduced significantly when
the aperture is stopped down too much, especially at close-up distances.
A working aperture of f8 is recommended. If all the settings are
recorded, there should be no difficulty in establishing the correct one,
based on the negative that is judged to be the sharpest, after the film
is processed. A new index can now be marked on the depth-of-field scale,
opposite the lens' original infinity symbol, indicating the true
correction. Remember that the new setting is valid only for the film and
filter combination (and the aperture) used during the tests. 

The newly-established index shifts again in the close-up range, because
the supplementary extension of 1/200 to 1/400 that must be added to the
normal lens extension has to be taken into account. A different
correction is required for every reproduction ratio. The same procedure
can be used to find the new index. The correction established earlier
for subjects at infinity can be used as a starting point. At a
reproduction ratio of 1:1 the necessary correction is twice as large as
that required at infinity."
-- 
joe b.

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End of INFRARED-PHOTOGRAPHY Digest 14
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