I have a different approach to the same problem. I built a mask to put over the dark slide of a sheet film holder. The mask has 15 holes, each 0.5 inches square. I have a means to cover each hole independently. With the mask taped to the film holder, and the camera aimed at a fairly large evenly-illuminated surface, take the cover off of one hole, put the filmholder in the camera, pull the dark slide and make an exposure. Then replace the dark slide, take the filmholder out of the camera. Cover the first hole, uncover a second, etc. This is described in more detail on my website www.vsta.com/~alrob, Appendix E.(posted 8575 days ago)Al Robinson