Filters in front of the film plane - backgrounder

[ANDPPH@xxxxxxxxxxxxxxxxxx]

Permit me to jump in on this discussion since I believe it was possibly my
suggestion  many years ago that IR filters could be placed in this position and
allow one to use SLR cameras for "action IR photography". This appeared in the
PSA Journal. Even before this time this was "standard" operating procedure in
my Photoinstrumentation classes at RIT. In that same article the "modification"
and calibration of a Weston LunaPro CdS meter for more accurate IR exposure
predictions was also mentioned and had been covered in my "Special Effects"
classes for many years.                                                 
                               
Anyway, with respect to the location of the filter, it can be placed in  front
of the FP shutter mask but it can also be placed behind it. This is due to the
fact that the film plane rails of the camera are somewhat raised above  the
bodu casting. The wratten filter is so thin that it easily fits in this 
"channel" and does not touch the film ... at least not in my experience!

Looking form the side, at a cutaway section of the camera body, things look 
like this:

    =====================================================  < pressure plate
     |   |=========================================|   |   < film
     |   |   |   |=========================|   |   |   |   < filter
_____|   |___|   |_________________________|   |___|   |_____
                                             ^       ^film position/pressure
                  <-------- 24 mm -------->  film plane rails     /plate rails 
          <---------------- 35 mm ---------------->
                         
                         ^   ^   ^
                         |   |   |  Light + IR + UV (!)     

The  filter, about 24 by 45 mm in sizecan be held in place with thin two sided
tape or taped down with thin Scotch mending tape (Yes, there is even room for
a layer of filter plus a  layer of tape!).                      

"Disadvantages" of this placement are that any dust particles on the filter 
will cast a shadow of themselves on the film and that slight mismatch in size
between the filter and the film plane rails will allow a little light to leak
onto the IR film causing picture edges showing a fine line where there was not
complete coverage ... typical of this method. OTOH, with the filter placed in
front of the shutter these disadvantages become minimizeb BUT in certain
cameras that location is inadvisable since there may be moving mechanisms
associated with the mirror and meterin systems which could be compromised. Not
to speak of clumsy fingers! In addition, one the filter is "hidden" the chances
of using an IR filter equipped body for regular photography increase
substantially with often dire consequences.
By the way, if you want to check out (text only for now) article that described
this suggestion in the PSA Journal, you can find it on the web among several
articles that I've posted on the web and loosely associated with the PhotoForum
list and located at:

                    http://www.rit.edu/~andpph/articles.html
Andy
 o o  0 0 o   o  Andrew Davidhazy, Imaging and Photo Tech - High Speed Photo
  \/\/\/\/\/\/   andpph@xxxxxxx,  http://www.rit.edu/~andpph,   716-475-2592  
___|        |_______________________________________________________________
                 PhotoTech Department:   http://www.rit.edu/~andpph/ipt.html 
                 PhotoForum list: http://www.rit.edu/~andpph/photoforum.html 

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Topic No. 10