Re: Kodak HIE & 4x5 Film Holders - Reply

[Scott Donkin <photos@xxxxxxxxxx>]

At 03:58 PM 09/12/96 +0000, you wrote:
>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.

The measurement should be taken from film plane to Rear Nodal Point.

At closer focusing distances, the 1/4 of one percent can still be measured,
and, of course, will be greater.

Either way, the 1/4 of a percent is a guide only.  I have had fair success
using this method on 4x5.

>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.

Sure.  Most all lenses are like that.  But a 1/4 of one percent of the 2.7mm
(50 minus 47.3) is so small that it shouldn't cause you a problem.  Either
way, how many of us are shooting IR with the lens set wide open?

>I am not positive that large format lenses have the same *error*,

It is not "error", but the manufacturer's final measurement and
certification of the lens he has just built.  Rarely will you get a 50mm
lens being certified as spot on 50mm, no matter what format it is for.


Cya...

   ______
   Sco||                E-mail:   photos@xxxxxxxxxx
                       Fidonet:   3:713/914.11

..due to the current economic situation, the light at the end of the tunnel
has been turned off until further notice.


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Topic No. 14