Why is my camera out of focus after changing focus screens?

I shoot a Canon 350D and use some old Nikkor lenses… I tried to get better results by buying and installing a split-image focus screen and hated it. When I re-installed my original screen and I use Aperture priority and get the focus confirmation beep… The image in the viewfinder is out of focus… The shot is great… But the viewfinder is out of focus… Even when I adjust the diopter setting on the eye piece. Can anyone offer a tip?

If you don't get the focusing screen back in EXACTLY in the proper position, or if you slightly bent the bracket that holds it, the image in the viewfinder will be out of focus.

It's also possible that you mounted it the wrong side up.
Mate side should face downwards (to the mirror).

Another possibility. Do you know what the diopter is? It's the tiny wheel beside the view finder that adjusts the view finder focus. It doesn't effect the camera's focus. Try that while you're looking through the view finder.

With the diopter adjusted, the only thing I can think of is an inverted focus screen.

Until Minolta introduced the world's first full-featured Auto Focus 35mm SLR in 1985, the Minolta Maxxum 7000, the majority of photographers using a 35mm SLR depended on the split-image and matte circle seen in the viewfinder for quick and accurate focusing. I'm surprised that you didn't find it much easier to manually focus your Nikkor lenses on your 350D.

What you probably did when you replaced the original screen is install it backwards. Take it out and flip it over and your problem should be solved.