I had a feeling... I've taken off the prism and it looks extremely bright and even if I point the bottom at a light and look through it. If I then hold the camera to my monitor and look through the screen like a TLR it looks more like what I see through the viewfinder. So maybe it's the type E (just looked it up to be sure) screen and it's meant to look this way? One thing that's unusual (to me) is that it's not flat like the type E screen in my FE2. It's actually convex on topólooks more like the front of a lens. Seeing as how it's giving the appearance of vignetting I though maybe that could have something to do with it.
F3 screens have a curved, clear surface facing up to the view finder--the bottom is a fresnel surface. E screens transmit as much light as any other screen. Any chance of a misaligned mirror?