I am planning to do some (4x5 sheet) film testing and for that I would like to construct a "test rig". My idea is to (partially) sacrifice a sheet film holder by cutting a window in one of its' darkslides. I then plan to attach a step wedge covering the window. The procedure would then be: 1) Load a sheet film holder with the film under test. Use a unmodified darkslide to cover the film. 2) Set up the camera in front of an evenly lit surface. 3) Take a light reading on the surface and calculate the exposure such that the surface is placed into zone X (?). 4) Insert the film holder, remove the "plain" darkslide and insert the modified one with the step wedge. 5) Make the exposure, remove the modified slide and insert the plain darkslide. My idea is that I should now have a single sheet with exposures from zone X and downwards. Any comments on this procedure? My biggest concern is that with a test rig as described above, the step wedge will not be in perfect contact with the sheet film, but there will be a small gap of approx 2mm between the film and the darkslide/wedge. Will this gap cause too much scatter and stray-light etc. for the approach to become improper? All comments welcome!