My hypothesis: the split-image rangefinder or the microprism crown rangefinder are not influenced by the defects of your sight, focusing on the ground glass is, your sight is not optimal, which makes you focally-challenged when using the ground glass.
For instance, when the split-image rangefinder shows a perfectly aligned edge in the two halves, the image is in focus. If you have sight defects you will not see the image perfectly in focus, but you will see that the two halves are aligned, or are not, so that the indication of the split-image rangefinder will be correct regardless of your sight quality.
Focusing on the bare ground glass on the other hand is influenced by your sight. If your sight is not the one which is presumed to be by your viewfinder (normally 0, sometimes -1 dioptre, it's indicated in the camera specifications) the entire focusing system will act as a correcting lens for your eye, and the best focus for your eyes will not coincide with the best focus on the film plane.
A problem with the camera mirror would normally affect both the split-image rangefinder and the bare ground glass, I suppose, and so would a problem of geometry between ground glass and focal plane.
Three remedies to the problem above:
- Only use focusing aids (split-image rangefinder, microprism-crown rangefinder) without relying on the ground glass (not satisfactory);
- Use your glasses when using your camera (your camera will scratch them unless the glasses are made of real glass, or you put some sort of rubber ring around the ocular);
- Mount on the camera ocular a lens of the correct power so that the sum of the power of the viewfinder and the power of the additional lens equals the power of your glasses.
For instance, supposing you have myopia and in your right eye, which you use for focusing, you need a lens with -2 dioptres, and supposing you camera, as it comes out of the factory, has a viewfinder with a -1 dioptre, you need to add a -1 dioptre in order to reach -2.
As far as I understand there is a little trick here to be kept in mind.
If you buy the original make dioptre for your camera, in the case above, the dioptre marked -2 will actually be a -1 dioptre, because the original manufacturer will probably take into account the power of the viewfinder and give you a dioptre marked with the "total effect".
If you buy a universal dioptre (or have it made by your optician) you have to apply a -1 dioptre in order to obtain -2.