How Far is infinity?

Although it may seem like a philosophical or silly question, it is indeed a practical question for me. You see, we have a tradition at Christmas-time in my family where we play dominoes on the days before Christmas and whoever wins gets to open a gift. I’m sure I mentioned this around this time last year. Well, a couple of days ago I won and my prize was a Canon 50 mm f/2.5 Macro lens from my wife! It’s a fine lens – I’ve been taking some awesome pictures which I will be uploading once the Christmas Season is over.

This is my first Canon lens outside of the kit lens my 350D came with. I really enjoy my Tamron 55-200 mm, but the Canon lenses are truly worth what they cost. The quality is top notch and it has a focus area where, as you focus, it tells you how far away the object can be and still be in focus. It goes all the way down to tenths of a foot. But on the other end is infinity. That’s right, I can focus on infinity. Silly, since I can’t see that far.

Of course I’m being facetious; any photographer worth his trade knows that focusing on infinity means that everything will be in focus from front to back. (roughly speaking, that is, because everything has to be at infinity or farther) It’s a setting most often used for landscape photography, but on this lens, it tends to be good for a few feet away.

So the reason for my question is this – let’s say I want to prefocus. I may want to do this if I know that I will be taking pictures of something at a fixed distance. I don’t want to potentially miss a picture because the focus is hunting for the right setting. This can especially be a problem if it’s dark or under certain tricky lighting conditions. So if I want to prefocus at infinity – how far should the object be? Is there a way to tell?