Questions about how to use prefocus on an FD telephoto lens

[loccdor]

The Canon New FD 500mm f/4.5 L has two rings with focus markings - one is a prefocus feature. It has a thumbscrew that can be loosened and tightened.

If I want to take a picture of the moon some night when the conditions are right, how should I use this prefocus to lock in focus now so I don't have to spend time trying to achieve critical focus on the moon and possibly miss the shot? Does it actually work long-term for critical infinity focus? Depth of field is extremely narrow on this lens and infinity focus is very tricky.

 

[wiltw]

Shooting the moon, simply set the lens on Infinity...no need for focus lock!

Even using the more-precise 20/20 vision acuity standard for DOF calculation (vs. 'manufacturer standard'), 500mm FL focused at 10,000m has DOF at f/5.6 of 3,000m - Infinity!
(Focused closer, at 1000m the same combination has DOF from about 800m-1200m, so much more restrictive.)