Q.G. de Bakker , Jun 29, 2005; 09:00 a.m.
First, the screen isn't opaque black, but transparent so it will not interfere with light meters in prisms.
There are opaque black ones too, which do have an effect on the meter reading.
The mask is indeed used for both 4x4 "superslide" and 6.5 x4 formats. Hence the "lines all over it".
A Hasselblad viewfinder image does not show 100% of what will end up on film, no. It's a few % less. You can easily test this yourself: measure both the distance between the metal edges of the focusing screen, and the width of the negative/positive. You must have both, and a ruler too, so you should know before anyone has a chance to answer. ;-)