The Canon EOS 10D, like most modern autofocus cameras, has a viewfinder screen that leaves somewhat to be desired for fine manual focus. Manual focus is still required for special applications like macro photography or the use of Canon’s TS-E Tilt-Shift lenses.
The only way to improve the accuracy of the laser matte ground glass is to use a focusing magnifier. One is built into the Angle Finder C, but that is a very expensive accessory (not quite as princely priced as the $250 Leica Viewfinder Magnifier M 1.25x, however…).
Another, cheaper option is to use the Canon Magnifier S. This accessory has been discontinued by Canon, but it can readily be found on places like eBay. I paid $56 for mine (mint “old new” stock), including shipping.
This is a focusing loupe that slides onto the 10D’s viewfinder using the supplied Adapter S as a replacement for the standard eyecup, as shown below. It magnifies the central portion of the image only by a 2.5x factor. Unfortunately, it seems Canon does not make a wide-field magnifier equivalent to the DW-4 viewfinder for the Nikon F3, which offers 6x full-field magnification!
The advantage of this setup is that the magnifier can easily be flipped out of the way as needed.
I got to handle the new Canon EOS 300D (Digital Rebel) last Wednesday, and the Magnifier S fits it as well.
Yes, it also fits the Rebel XT.