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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?

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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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    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
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    @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
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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
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If you shoot infrared, the "infinity" point for IR is actually farther than the infinity point for visible light. A lens that did a hard-stop at the infinity point for visible light wouldn't be useful for IR imaging.

Similar... red, green, and blue light focus at different distances and if you use filters (especially narrow-band filters) you would want to be able to compensate for this.

There's a little margin for error in each copy of a lens or a camera where the sensor might be shimmed fractionally closer or fractionally farther.

There are even thermal expansion properties that can shift focus ... metals contract when they get cold.

So there's no single reason, but rather a lot of reasons why it's a good idea to give the lens a little wiggle room. And think about this from the financial standpoint of the lens maker... if they did a hard-stop and for whatever reason that "copy" of the lens happened to be just shy of focus on your particular camera body copy then that lens has to be returned as "defective". But with the wiggle room... that saves them a warranty claim.

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