one thing nobody has mentioned is that the sharpness of focus also depends on how fine the ground glass is ground -- it can look sharp when it isn't really.
Leica used to say that the way to show how accurate its rangefinders are was to put two pins stuck into the table 3 feet away, one pin an inch in front of the other. If you focus with the rangefinder on the one in front, you will notice that the one in back already looks a bit doubled in the rangefinder.
An SLR, I wager, would make both pins look sharp except for the most excellent viewing screen around (leicaflex SL2???) but the real point is that, with a wide-open lens, both pins can look sharp in the viewfinder, but the image will show a difference.
this is why a good rangefinder is better in dim light, especially with wide-angle lenses. It may not be a defect in your slr, it may just be the way the system works.