Chris Stoddart wrote:
Also I can't see any concavity/convexity on the filters and they don't seem to alter the image when you look through them...
It's very very slight. I could only deduce it by looking at the reflection of a light fitting, as I described. At the very least the filter has to affect the focussing of the lens slightly, since its increased optical depth wrt the air it replaces will marginally increase the backfocus.
S
