Originally Posted by newcan1
I agree with you. It seems intuitive that a split-image focusing aid would be more precise (and accurate), but I think that this is wrong. My practical experience indicated that ground glass is more reliably accurate, as per a focus test using about a dozen different people.
I won't go into details, other than they each focused a piece of test gear, one time using a split-image screen, then using only a ground glass. Everyone felt they had nailed it with the split image, whereas the ground glass seemed more uncertain. The results, based on marking the lens barrel position, showed the opposite to be true. The ground glass focus barrel marks were all tightly grouped, the split-image focus marks spread out more than twice as far.
The results were so surprising that we repeated the test (an industrial decision was riding on this). The results were the same. Clearly, something odd was going on with the split-image system. Nothing changed except the person doing the focusing. We presumed that some difference in eye position was probably to blame, but did not investigate further. (The test was over, for our purposes.)
This test was enough to convince me to use the ground glass portion, only, when best accuracy is needed. You never feel as certain that you have nailed the focus, but it seems that a split-image glass can give a false sense of accuracy. BTW, for practical purposes, only a couple of "outliers" would likely have shown up on film tests.