First, one should read and understand this technical paper:-http://dougkerr.net/Pumpkin/articles/Split_Prism.pdfNext, we should learn that every lens has spherical aberration, this is the chart for the luxury Zeiss Planer 85/1.4 (which was available in ZK Mount):-via the Chinese Xitek Pentax ForumTake this as an example, we can see the maximum deviation of focus can vary as large as 0.13mm, i.e.,
