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Originally Posted by Jonathan
Hi Roger. One reason for this is that ED (Canon call it UD I think) glass changes size slightly with temperature so the position of infinty on the focus ring can change depending on the temperature. Only 1 of my lenses stops at infintity and it's a non-ED glass lens, the rest go past infinity, the longer the focal length the further past it goes.  |
Hence the need to check focus throughout the night if your imaging all night.
It's not a basic set and forget.
As temp changes so does focus.