These are images taken through the viewfinder of an old film SLR with a spilt prism focusing aid. A sheet of paper with repeating lines is being focused to demonstrate that the lines can be "lined up" but shifted one row which results in a severely out of focus image.

The first image is just slightly defocused to show that the split prism clearly indicates the defocus condition.

The second photo indicates the proper focus condition.

The third photo indicates how the lines can be apparently matched but the image is severely out of focus.