Re: Kodak HIE & 4x5 Film Holders - Reply

[KEN SINCLAIR <SINCLAIR@xxxxxxxxx>]

I have seen some reples to postings about the *focus* shift for IR
whre the shift is !/4 of one percent of the focal length of the
lens. IF you are focussing at a distance of less than infinity ,
you have to take into consideration the TOTAL distance from the
lens to the film, not just the focal length of the film.
Now, since many lens makers indicate that their lens is of a
particular focal length, we cannot accept that that is an accurate
figure. If one *browses* some of the photographic magazines at the
favourite corner store  (that is the ones that are not *really*
photography magazines but an advertising medium) you can see that
in some of the camera and lens tests that the actual FL of the
lens under test is measured on a collimator. In MANY cases the
lens is not 50mm but 47.3 or perhaps 52.8. In other, words the
manufacturer rounds it out to the nearest common FL.
I am not positive that large format lenses have the same *error*,
but I would bet that LF lenses would have the same +/- percentage
of accuracy. I believe that this could also affect focus *errors*
when using sheet film (those who have roll film holders add roll
film here 8-))
Notwithstanding the depth of focus *tolerance* I would imagine
that the chosen F-stop would help negate many of the slight
imperfections. Those with old Compur shutters cannot adjust the
f-stop quite as finely as those with Copals but we can get close.

Ken 

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Topic No. 8