400/5.6L switch

[Belmondo]

I just purchased a Canon 400mm f5.6L telephoto lens. It didn't come with any documentation, and I've figured everything out except one:

There is a two-position switch on the barrel for 3.5m to infinity, and 8.5m to infinity.

What is the purpose of that switch, and under what circumstances would I use the respective settings?

The lens is 'like-new' and takes pictures of amazing clarity. I'm starting to understand the L-glass mystique.

Tom

[Belmondo]

I neglected to say that the lens was used, although I insuated as much by the 'like-new' reference.

[Mark Kemp]

The switch is the 'minimum focus distance' in metres

Choose the smaller number and it will focus on closer objects.

Choose the larger number and it will focus more quickly on objects farther away.