manual focus/distance marks on lens/infinity focus

Hi all,

I've been learning to use the video function of the 5D2 and 7D and therefore been working on something I'd really never used before: manual focus. Now I'm a bit confused about something I had not thought much about or noticed before since I had relied on autofocus.

When I look at the distance markers on my focus dial, each lens has a line on it indicating where the maximum distance is - where everything beyond that measurement is in focus. (Which is great and very useful to know and I wish I'd realized that sooner, with back button focus I realize I don't need to refocus a lot of the time.)

What I don't get is that the you can continue to turn the focus dial a little bit further beyond that point. When you do, it looks like nothing is in focus. This is really annoying, and I don't get the point of it! It seems like it would be so much more practical that when you turn the lens as far as you can go, you're at the maximum focal distance. Instead there seems to be a small sweet spot that you have to hit (and on some lenses, like the 100mm macro, that sweet spot is really small).

Am I missing something here? Is there a reason for this? Insight appreciated. Thanks!

I think that the extra space past the infinity marker is for the lens to compensate for the expansion/contraction when under extreme heat/cold.

Edit:
Here's the image pulled from the manual from the 70-200 2.8L II

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The better lenses have a lot of focus past infinity. You just can't turn it all the way over and expect to get pinpoint stars (infinity.) LiveView 10X can be your friend.

My belief is that it's to give the focus motor "room to work." In just about any servo mechanism which looks for some sort of null or solution, the usual practice is for the sensing mechanism to drive PAST the optimum point and back, in order to actually detect it.

It's mostly a thermal expansion thing.