Another factor that has emerged as I play with this - and that's the effect of the cooler box on the "distance-to-sensor" figure. If you're using an MPCC or some other reducer/flattener which has a tight tolerance for this - e.g. the 55mm of an MPCC - then there will be a problem. The T-Ring will have to sit in the camera and inside the box - meaning that there will need to be some form of external connecting ring. In the photos I've attached below, you'll see I have put such a ring in place. It screws through the box lid and into the T-Ring but at a cost of about 8-10mm added distance to sensor.
Peter
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