[markhail]

Leaving aside for now whether I should just hit the lens with a hammer and move on to something better (the deterrent to that idea is several teens in braces right now).....

I have a Vivitar 100-400 "Series 1" AF zoom which worked reasonably well when I first got it. Now, however, sometimes the autofocus doesn't. The lens is a screw-drive focus . When I manually focus there is a dry rubbing sound but a smooth one, as opposed to what one might hear if it had picked up some dirt. The lens has been tried on everything from K10 to K-7 . Curiously, the K20D in the to-be-gone bag seems to be able to drive the lens the best.

Given that I can get access to a Bigma, I still like this lens -- IF it can be made to work consistently -- due to weight. I take it to softball fields and , being often limited to shooting through the chain-link fences, this lens gets less cloud effect than the Bigma too.

I note that when one rolls back the big rubber grip there are three Phillips-head set screws, equally spaced around the lens barrel. This is probably a trap <g> so my first choice would be to see if I could (a) lubricate the barrel at the focus ring and the part of the lens that actually rotates and extends and (b) if so, with what. Some sort of light grease, perhaps ? (Graphite and WD-40 are obviously out.) Or should I "just" take the bit between my teeth, break out the screwdrivers and my dictionary of swear words , and see what falls out when I loosen the screws?

There is a replacement possibility at KEH but the discussions suggest that $168 is far too much to pay for the lens. I'm thinking that sending the lens off for cleaning is as expensive as the KEH alternative, so before I make any irreversible decisions......

[Adam]

I doubt the problem is that simple- I'd say you'd need to open it up to clean it up right and see what's really wrong.