david,
the holga has a square adapter, the vignetting comes from the long exposure. The negative is completely black after development. I then use a potassium ferric cyanide bleach. Each neg is tray bleached with no aggitation so it is almost like I am etching the silver. this allows for the interesting fog effects and the intensification of any holga type flaws. The best part of the whole thing is that these are very intuitive and so much the opposite of all the 4X5's that I have been working on for the past 10 years. these new guys are lots of fun for me. Thanks so much for all the comments.
michel