OK, so I decided not to derail this thread any further... I was going to reply to this post, but thought a new thread would be more appropriate. Now, on all of my lenses, the red dot for IR focus reaches infinity before the 'normal' focus mark. That means that you would never have to go past infinity to focus IR at infinity. pgriz says that it's to allow for temperature variances (and cites the manual to a lens). I said that it's to allow the AF mechanism to overshoot it and come back to prevent damage (just seems right - I don't have any references). So, we have 3 theories - (1) IR focus; (2) Temperature; (3) Built in 'slop' to prevent damage. Since IR reaches focus before visible light, there is no reason to go past infinity in IR. The temperature differences seem kind of small to me (relatively) for it to affect a lens to that degree. The 'slop' theory sounds good to me, but I can't back it up with anything. So, the question is - why will some lenses allow you to focus past infinity? Also, is there any situation (other than those mentioned) where it would be desirable to focus past infinity?