I take sort of the same position as AgX does - I'm astounded that two lenses Dan mentioned have t-stops implying that only about 30% of the light entering actually contributes to the image.

I don't doubt that Dan has to do what he says, based on his tests; my reservations are in the presumption that it is strictly a t-stop issue. Dan says, "If you haven't done the testing, don't grumble about my results." My counter to this is, I know how the standard (effective 1964 and 1969) says to test, and he didn't do it that way.

I can't say with any certainty what might be the reason for the large difference between f-number and t-numbers, but if I had to find out, I'd start by measuring (roughly) the focal length, then the actual aperture diameters. A discrepancy here could go a long way toward explaining the issues.

I'm glad I don't have one of those lenses, 'cuz I'd have a hard time sleeping 'til I found out why they work that way.