Paul, you say its a simple rule of thumb accepted since Nikonos times, and i was just wondering why it was accepted and why we had to do these things. Just because our forefathers said it was true doesnt mean we cant question it, otherwise our world would still be flat and we'd be holding our breath instead of strapping tanks on!
Well whilst our view of the world has changed over the last few hundred years, optical theory and practice hasn't changed so much since the 1950s. If anything our digital cameras are far more likely to show up the corner problems which limit the use of a flat port. If you try using a wide-angle behind a flat port you will find that the corners show a range of problems until full internal reflection is achieved. This has not changed. The rule of thumb still applies.
In fact Cousteau had domes ground which were not concentric (in effect aspheric ports) in order to produce better image corners back in the 50s too. We have a wider range of glass available today but no dome manufacturer of consumer products yet builds an aspheric dome.
