What is meant by optical distance to the CCD?
The optical distance takes into consideration the index of refraction of any glass that may be in the optical path. For instance, nearly all cameras has some sort of cover glass used to seal the CCD chamber. The thickness of this glass may vary from manufacturer to manufacturer (and even between models of the same manufacturer) and must be compensated for in the mounting plate. Basically, the introduction of flat glass into the optical path will increase the optical distance to the chip. In other words, even though the CCD may be a known physical distance from the faceplate, the optical distance will usually be further. It is this optical distance that we are most concerned with.
