I think I read somewhere that the Kaiser VP6005 was a hybrid diffuse/condensor light source, so it may be that your putting a piece of diffuse material in the lightpath is actually restoring it to its original spec! Beejaybee is also right about lenses to some degree causing a slight vignetting of the image. Generally speaking, the best lenses (i.e. better constructed, more elements etc) reduce vignetting and other optical maladies to a minimum. This makes them more expensive. This also applies to enlarging lenses. If you're going no bigger than 10x8 prints, a Meopta Anaret lens will do the job well, especially if you stop the lens down. Trouble is, if you're going bigger than that, you'll need all the light you can get to avoid excessively long exposure times. Opening up the aperture will show the vignetting again. This is when a good 6 element lens comes into its own. This is also why it is always best to buy the very best lens you can afford. Luckily, at the moment, there are lots of bargains to be had with enlarging lenses.