I talked with my dealer to prepare the equipment list for the move to mono, the 460ex. As we were determining the backfocus for the spacer requirements, he told me that you need to add for the filters. He said 1/3 of the filter thickness. The Astronomiks list their glass at 1 mm, which is inconsequential. But my dealer said they are 3mm hence an additional 1mm backfocus. This is something I haven't read about before. 1. Has anyone heard of filters adding to backfocus? 2. Is the 3mm for Astro