[Toshi]

I have the 50-135, and the AF failures with my lens can be directly fixed by cleaning all the contacts very thoroughly. Some of the lenses seem to be just a tiny bit fussier than others about the contact cleanliness. Sometimes removing and replacing the lens is enough, at other times I have to use a micro fibre cloth and alcohol. It's worth the effort to have this superb lens in my arsenal.

Thanks for the input. After posting this thread, I did read about cleaning the contacts helping another user. I've decided to take the plunge and just finished placing my order

[Hannican]

Where'd you order from? I'm considering placing one too, but want to make sure I get the best deal possible.... I need to make sure I have a good multi-year warranty on it too...

[Toshi]

I'm not sure if it's the best deal, but I purchased mine from Adorama since I've ordered from them before and have had no problems. It's 699.95 with free shipping (but I upgraded the shipping). Warranty is standard manufacturer 1 year though.

[Hannican]

Thanks Toshi!

[nulla]

I agree with you Jay

There is no comparison between the two - the 16-50 has (had?) a relatively high failure rate, the 50-135 doesn't.

Every single lens line manufactured is going to have a few lemons, however, so that's probably what you're hearing about.

The Tamron 70-200 2.8, while exceptional, has it's share of QC issues too - I had to send one back because of AF problems myself.

Overall, the best guarantee, IMO, of exceptional IQ in currently-made K-mount s the DA* 50-135, DA 12-24, FA/DA 35's, DA 70ltd, DA 40ltd, FA 77ltd, FA 31ltd, and FA 43ltd.

I also have the Fa 77 and the last 2 on your list are my next 2, be it next week or next year

[paolojackson]

Had a scare today with my 50-135. Just got a K20D off the forums and I had the lens on. Wouldn't focus. Flipped the AF/MF switch on the barrel a couple of times, and it starts working again. I fear that eventually my lens will suffer a fate similar to others. But that won't stop me from using it. This is an interesting conversation, one that I've stumbled upon more than once. Is the Pentax SDM system just inherently flawed?

Also, what is a good way to clean the contacts?

[attack11]

Had a scare today with my 50-135. Just got a K20D off the forums and I had the lens on. Wouldn't focus. Flipped the AF/MF switch on the barrel a couple of times, and it starts working again. I fear that eventually my lens will suffer a fate similar to others. But that won't stop me from using it. This is an interesting conversation, one that I've stumbled upon more than once. Is the Pentax SDM system just inherently flawed?

Also, what is a good way to clean the contacts?

the af system works when it's got proper electrical contacts. anything else is user error or a defective product.

what you described is a bad mounting. i did it once, just pop the lens off and re-seat it.

[Canada_Rockies]

the af system works when it's got proper electrical contacts. anything else is user error or a defective product.

what you described is a bad mounting. i did it once, just pop the lens off and re-seat it.

And if it happens too often, the micro fibre cloth and alcohol will resolve things nicely. I suspect that the contacts in the lens switch had some minor oxidation that flipping the switch back and forth cleaned up. You did exactly the correct thing when you flipped the switch a few times.