Re: Singer RH70 Back Questions

[Clive Warren <Clive.Warren@xxxxxxxxxxxxxx>]

At 6:43 pm -0300 22/6/00, Eric Boutilier-Brown wrote:

>Hi all,
>
>I have picked up a Singer RH50 back, and am about to begin arranging to work
>with it on my Wisner 4x5; I have an IR bellows on the way, and will be
>building a new back-frame to house the back, but before I actually put some
>film through it, I have so questions, in the hopes that others on the list
>have used an RH50 back for 70mm IR.
>
>The main question is concerning the pressure plate - it has a pattern of 10
>holes in it - does these show up on the IR negs? I suspect they will, as the
>pressure plate on an EOS 3, which has a hole cut in it for a data back,
>causes problems. If it will show on images, what are my options? Fill them
>with epoxy and paint them black? Is there a reason for holes in a pressure
>plate, and if so, am I going to have a problem if I full them in?
>
>Any and all help would be appreciated.
>
>Take care,


Eric,

I have not used a Singer RH50 back, however may be able to answer some of
your questions.

The "holed" pressure plate effect is due to areas of unequal reflectance of
infrared light which has passed through the film and then is reflected back
into the film from the pressure plate. As you probably know, HIE does not
have an anti-halation backing so is prone to this sort of effect, and
indeed some people would say that the resulting blown out haloes are one
aesthetically pleasing property of the film.

If the pressure plate has holes then infrared light will not be relected
back equally across the rear surface of the film. This may or may not be
noticeable depending on how you expose your film.  The more infrared light,
the more noticeable the effect on areas of continuous tone.

There are an number of ways of guarding against the unexpected appearance
of "crop circles" in your resulting prints. All methods require that the
film pressure plate reflects infrared light equally across its surface. The
easiest solution is to use the backing paper from 120 roll film, black side
towards the film. Cut a length long enough to go around the pressure plate
and tape the ends using Scotch tape. Another solution is to have someone
fabricate a replacement pressure plate which has no holes in painted steel
or black anodised aluminium.

It would probably be difficult to match the reflectance of the black
painted epoxy filled holes with that of the main pressure plate, so this
would not be a good solution.

The holes in pressure plates are there for a number of potential reasons
depending on the camera. They could be for access points to
screws/adjusters, to reduce weight, and on a flat pressure plate to reduce
"stiction" ie to allow the film to pass over the plate more easily.

Hope this helps - a lot of the above is in the Infrared FAQ (URL in sig.
file), together with a section on use of 70mm in case anyone else is
interested in the idea of using 70mm HIE.





All the best,

             Clive   http://www.cocam.co.uk
                     Photographic Services, Filters and Equipment, Infrared FAQ


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