Why am I allowed to turn my focusing ring beyond infinity?

Is there a good reason why I can turn the focusing ring on my lenses beyond infinity mark on focusing scale? Following recent questions here and here, it sounds like a real usability issue.

Clarification: 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?

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This question seems nonsensical as focused beyond hyperfocal seems self-contradictory. Can you elaborate? –  msw Aug 12 '10 at 13:04
I think the real question is: where is infinity located, and how can there possibly be anything located beyond it? ;) –  Fredrik Mörk Aug 12 '10 at 13:11
@Fredrik Mörk: I don't know. But if there is something there, it sure would be handy to be able to focus on it. ;) –  Guffa Aug 12 '10 at 15:03
msw- Hyperfocal distance is the closest point in which everything beyond the subject is within the depth of field. Infinity distance is farther than the hyperfocal distance. Are you are asking why bother with anything beyond hyperfocal distance, as that includes infinity? I assume the answer would be that the photographer wishes to start their DOF beyond the point it would start when focused at the hyperfocal distance. –  Henry Peach Aug 13 '10 at 0:52
Frederik- The human perception of infinity, and where a lens finds infinity are vastly different. I have lenses that think infinity is 30' out. ;) –  Henry Peach Aug 13 '10 at 0:55

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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+1 for mentioning that focus varies on zoom lenses as the focal length changes. –  Hamish Downer Aug 12 '10 at 15:50
Good point about the zoom lenses. My 70-200mm really has very different understanding of "infinity" at different focal lengths. When I use autofocus and focus on clouds at 70mm the focusing scale stays somewhere between 10m and infinity mark and at 200mm, it really goes to infinity mark. That means I actually can't trust the focusing scale for manual focusing at focal lengths below 200mm. –  Karel Aug 12 '10 at 15:55
@Karel True in a sense, but I think you can trust it as much as you ever did. I.e., if you were actually scale focusing, there are enough other sources of error that this one is unlikely to have a significant effect. –  ex-ms Aug 13 '10 at 0:22
@matt - no, I don't scale focus the 70-200, maybe just the 17-40 sometimes. But if I'd do that, infinity would probably be the only mark I'd really need to be correct. –  Karel Aug 14 '10 at 18:58

The manual focus lenses I've handled (M42 Takumars) actually have stopped right at infinity; it's the AF lenses that have slush beyond. My understanding is that there's two reasons for this:

1. Conditions (mostly temperature) can move the focus point slightly
2. The autofocus system needs to be able to "miss" infinity a bit without slamming into a mechanical stop, which would cause excessive wear.
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I also thought about AF calibration, but I'm not sure whether the calibration is a constant offset or depending on distance and it's still should be very small amount. –  Karel Aug 12 '10 at 13:33
It's less about calibration and more about simple error. Say your AF system misses focus 1 out of 100 times and thinks it needs to go beyond infinity. You don't want it to hit a hard mechanical stop that often (or ever, really). So you could either design fancy electronics that need calibration to detect this situation and initiate a smooth stop that ends exactly at the limit of travel (infinity), or you could add a little extra travel to absorb this error - extremely simple and needs no calibration. –  Reid Aug 12 '10 at 15:03
I understood your points and my thought regarding calibration was meant as an addition to your list. When AF focuses forward of the subject and it's calibrated to focus a bit further, it might need to go beyond infinity mark also. I've never had my lenses calibrated really, so I don't know how it exactly works. –  Karel Aug 12 '10 at 15:41
+1 i have the exact same experience and opinion :) –  JoséNunoFerreira Apr 5 '11 at 19:17