Originally posted by Canada_Rockies The A 70-210/4 is also rather good, and has f/4 all the way to 210mm, which can be an advantage compared to the F 70-210 which is f/5.6 at 210mm. That gives one more stop of shutter speed. The tradeoff is that it is a manual focus lens.
To my mind, the big advantage of that extra stop is the extra brightness you get in the viewfinder. I don't think I ever shoot any of my lenses wide open, but having a nice bright viewfinder is what draws me to a fast lens.
I guess the other tradeoff of the A lens would be that you have to manually enter the focal length if you're using the SR. Doable, but limiting in some situations. I'd much prefer the build of an A series zoom over the F, especially WRT the focus ring. I own the F 70-210, and I don't like the dim view through the viewfinder and the crap focus ring. But it's nice to NOT have to enter the focal length.
One thing to be careful of is that I don't think all the A series 70-210/f4 lenses are created equal. I think there are some "Takumar-A" versions whose coatings aren't up to the usual Pentax standards. I played around with one once, and it was very prone to flare - bright scenes were often washed out.