Is there a good reason why I can turn the focusing ring on my lenses beyond infinity mark on focusing scale? Following recent questions about focusing in the dark (here and here), it sounds like a real usability issue.
When I'm focusing manually, I only care about the infinity mark on my focusing scale, but there is around 5-10mm of room (depending on the lens) beyond infinity mark. Why is it there?
Answer
The most obvious reason is to be sure that you can reach the spot where it focuses at infinity. It would be hard to make the lens stop at exactly infinity, and any little change (temperature, humidity, filters, et.c.) might move that point slightly, making it impossible to focus exactly at infinity.
On a prime lens you would need only a small margin, so they can be adjusted to stop just slightly beyond infinity. On a zoom lens the focus varies somewhat depending on the focal length, so it needs a lot more margin.
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