The problem with focusing screens designed for AF cameras is the way the screens are optimized. AF screens are designed for clear, bright viewing, and have a very smooth texture to their grind. The practical effect is that the depth of field shown does NOT, I repeat, DOES NOT, represent the shallow depth of field of apertures that are physically wider than about f/4.5 or thereabouts. So, wide-aperture lenses which have very shallow depth of field, and which need critical, spot-on focusing, are actually somewhat handicapped when the shooter tries to manually focus them using a screen that is optimizwed to be bright, clear, and easy to view.
The problem of manually focusing AF lenses is even worse on crop-body cameras, since the image is small, and many lower-end bodies use pentamirror instead of pentaprism systems, and the overall through-the-camera viewing experience on the intro level bodies is pretty far from what it is on the flagship-end bodies. With my glasses and my vision, I find the low-end Nikon and Canon bodies and their viewfinders absolutely abysmal. Offsetting that is the fact that the mid-level bodies and up now offer live view, with instant zoom-in, and so on. Still, many AF lenses have VERY short manual focusing throws, which can make it difficult to hit the exact, precise distance needed when a creaky old human is doing the focusing,m and not a modern, high-speed, computer-driven mechanism.
Anyway...a manual focusing screen WILL look a bit dimmer, and coarser, but will offer more contrast, and will show more-accurate depth of field, that is much closer to what a wide-aperture lens will show, due to a different degree of "scatter", which is determined by how the viewfinder screen is designed and made. Different viewing screen types often require an exposure "offset" to be entered, to compensate for different screen brightnesses that deviate from the expected normal screen's baseline. My guess is that the OP might have better luck, better results, focusing an 85/1.2-L using the Rebel's AF system. The lens is optimized for AF, and the camera is too.
