Originally posted by jms698 Pardon my ignorance, but isn't the moon, for all intents and purposes, at "infinity"?
It is but finding exact infinity in a lens is very difficult.
1. Wide open, fast lens = miniscule DOF.
2. Temperature variations change the position of infinity on the lens. A, say 25 degree Celcius variation in temperature makes a difference.
3. Many lenses focus past infinity. All super telephotos and the eg the FA Limiteds. This to compensate for thermal contraction/expansion.
4. Precision in the mirror finder system is never 100% and will vary from camera to camera.
5. True focus or ever so slightly off is hard to see on a focusing screen.
6. Precise focusing in low light, infinity or not, is virtually impossible.
The only way to ensure absolutely focus is on the film/sensor plane.
High magnification live view will be a very useful to astro and aurora photographers.
Some examples:
Here I stopped down the 77 Limited to F:3.5 in order to be certain infinity was in focus:
Red Aurora. Raftsundet, ...: Photo by Photographer Pål Jensen - photo.net
Here I stopped down either the A 24/2.8 or the 31/1.8, can't remember wich lens I used, to F:4 for the same reason.
Aurora Borealis in moonlight. ...: Photo by Photographer Pål Jensen - photo.net