Quote Originally Posted by Murray@uptowngallery
Is the groove for light trapping only, or is it part of filmholder retention too?
Both, but I can't tell you what is te most important one:
- the ridge will make sure that the filmholder sits at the appropriate place in the camera back, i.e. compared to the ground glass on which you compose the image.
- since a filmholder without the ridge feature an open end at the side you insert it in the camera back, it would be prone to light leaks, even with velvet light trap.

G