Originally Posted by dlacouture 
Focus screens are in fact no longer just ground glass, but are now fresnel lenses... Those fresnel lenses are designed to optimize light transmission for given apertures, and so are not aperture-linear anymore.
I may be wrong here, but I don't think the Pentax focus screen is a fresnel lens, just a ground glass. On a fresnel lens, you can see concentric rings on the glass itself with a magnifier (or regular pattern like diamond shape), and I don't see any on the screens I have on my Pentax. And I used to have fresnel screen on my Olympus and Konica in the 70's, so nothing is new here. A lot of times, focusing screen were manufactured with a fresnel pattern in the center to make focusing "pop up" when it was dead on.