70mm Pressure Plates and IR

[Jim Henderson <hendersonj@xxxxxxxxxxxxxxx>]

-- [ From: Jim Henderson * EMC.Ver #2.3 ] --

Dear IR List readers (George Ling):

Your missive to the IR List found its way to me and I am replying to your
request for information about the IR reflectivity from the 70mm back's
pressure plate.

The plate does indeed reflect IR.  The small screws recessed into the plate
do NOT.  The consequence of this is that if you photograph a light object,
e.g. skin tones, and underexpose slightly, you will notice a circular spot
on the negative that is 1/3 f-stop lighter than the rest of the same area
on the negative.

There is only one fix to this that I am aware of.  

You must carefully mask the entire inside of the 70mm carrier except for
the pressure plate.  Spray the plate  with a photographic black paint--
preferably a satin finish to prevent scratching.  Rust-o-leum black primer
is good, but comes only in a matte finish.  

Coating the plate with a totally absorptive black paint will eliminate all
reflectivity.  The consequence of this will probably be that the film speed
will be reduced, because you will effectively eliminate all halation which
currently passes through the film base, bounces off the pressure plate, and
re-exposes the film.  One of the problems of no anti-halation backing that
causes problems.

I have not painted my own Mamiya RB 70mm back; however, since I have two of
them, I am planning to attempt to spray paint the plate and see if this
corrects the problem.  I would like to point out, however, that this is a
minor problem that one observes only when very light areas of the scene
fall onto the screws.  

I can normally eliminate this in the final print by simply dodging the
affected area.

Hope this helps.

Jim Henderson
Applied Scientific Photography

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End of INFRARED-PHOTOGRAPHY Digest 224
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