Charlie,

Have you tried using the standard technique of zooming in to max telephoto, and then set focus? You may then zoom out and your subject will remain in focus without any further adjustment. Of course this assumes leaving the focus control in manual.

true Pete but...
 

The minute the skater moves from the point of "critical focus" as you have described, the shot is now out of focus. Since skaters are shot close to ice level, the movement is just too much to maintain a rack focus.

Yup, what Jimmie said. Skaters have historically been very uncooperative about skating only on my preset focus line. ;-)

So to recap, sounds like my best bet is Av mode with the lens closed down as far as possible and the gain set on +6.

Anyone have much experience playing with black stretch?

-cw-