Hi Doug I have asked a few other folk about your backfocus spreadsheet, in particular in regard to the modification of the available backfocus when selecting a filter. We seem to be in agreement that when you add a 3mm filter to your system, it extends your backfocus by 1mm, but if I select a 3mm filter in your spreadsheet it subtracts 1mm. I dont think this is correct, can you clarify? Martin
@Martin Pugh I had it right the first time. The spreadsheet calculates the total back focus required by the camera. That subtracts from the back focus of the instrument. The filter increases the available backfocus, so it should be negative in the spreadsheet. Double negative!