Just FYI. Don't assume, as I did, that you can measure back focus of your imaging train by using a depth gauge or other measuring device by physically measuring from the glass covering the chip and through your filter wheel , spacers and etc. I did this assuming that the glass over the chip was adding only 1-2 mm. In the case of my 460EX, the protective glass sits more like 8mm above the chip. Instead, take ATIK's word for it that the camera's back focus is what they say it is (13.5mm for the 460EX). You can use a depth gauge or calipers to physically measure the rest of the train, but not through the camera.