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I have just found this statement in Wildi (4th, revised, p.125):
"The image seen on the ground glass of the Hasselblad camera, or the groundglass back when attached to the rear of the camera, corresponds to 91% of the area covered on the film with the waist-level finders, and 88% of the area for the PM5 versions of the prism finder. This smaller area coverage is done purposely, as the slide mount covers up a small area of the image on the film."
This is considerably less than 100%, and I am rather surprised, and shocked, to this out.
if I am reading that correctly, it is stating that the viewfinder shows 100% of the recorded image, but only shows 91% of the FILM. Obviously the film won't be 100% covered by an image, because 10% of it is blocked by the slide mount...
well, I made some pinhole cameras, and have used a couple "toy" cameras that DID use the entire piece of film to make the image, so it can make sense in certain situations. Besides, I could've sworn that I read somewhere that the hassie screen shows 100%. So with this information in my memory, the only logical conclusion would be that the unused section of film was being referred to. If the hassie screen doesn't show 100% like I thought, well, then the above statement means something else (such as what you mentioned).