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04-11-2007, 12:46 AM   #1
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Can't lock focus in DA 50-200 with K100D

While I enjoy the DA 50-200mm very much, I am not sure if I get a defective len. I can repeat the problem in NOT getting a focus lock at infinity distance, and I can reproduce the problems consistently at 50 - 80mm zoom range, more happening at 50mm. If I aim object in a far distance, I can't get a focus lock at 50mm. If I zoom it out to over 80mm, the problem goes away. And the strange thing is that it does not happen 100% of the time, more like 90% of the time. It seesm after a successful focus lock on inifnity, the subsequent shot at the same distance will not have AF lock. Shutter release is disabled when I can't lock focus.

If I shift from AF to MF in the zoom lens, I see two instances with eratic showing of red focal point even if I don't hit the shutter release. Other times, the MF seem to work fine with confirmation of focus lock of red focal point.

I am using Select for AF option with center focal point. And I tried both Auto and Center focus and the problem remains. And I confirm that other non-center focal points have the same focus lock problem at 50mm. Also I tried replacing with two sets of new battery -- nimh and akaline and the problems persist. In similar scenes and focusing distance and zoom at 50mm, when I use the kit lens, I don't see the focus lock issue. Any advice what I need to confirm the problem before I call up BHPhoto for returning the lens.

Thanks,
Hin

04-11-2007, 12:55 AM   #2
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Corresponding thread in dpreview

Can't lock focus on DA50-200mm, please help.: Pentax SLR Talk Forum: Digital Photography Review

And I am still not sure if my problem is in the lens or the body. I read couple of posts from Rise High and I hope he can help pinpoint if my problems. Should I return the lens or should I return both. I only have few days left. Or do I count on the Pentax Service Center for assistance?

Thanks,
Hin
04-11-2007, 05:08 PM   #3
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Have you tried using a different 50-200, say in a shop, to see if the problem repeats with another lens? If not then the lens is faulty.
04-11-2007, 06:33 PM   #4
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Ask Pentax for help.

04-12-2007, 04:42 AM   #5
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Well, this was my solution to the problem and folks can feel free to jump right in and tell my why it's wrong - I certainly won't be offended. I got sick of this not getting focus lock on what I thought should be easy stuff. I switched my camera from AF-S to AF-C and have never had the problem come back. I've just left it on AF-C (K100D). I thought I would end up with a higher percentage of bum shots, but it really hasn't worked out that way. The 50-200mm kit lens is my daily driver, btw.
04-12-2007, 08:23 AM   #6
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AF.C or 'OK to cancel AF'

Yes, the AF.C seems to address the problem a bit. But not having the focus confirmation is very unsettled for me. I don't know how often it happens to your case. For my da 50-200, at 50mm with a far distance object with focus ring in infinity focus mark, it happens very often, roughly 70% of the time in few days of test shots.

The other way to keep using AF.S mode is to customize the 'OK' button in one of the customized option related to AF in k100D. The option has a weird title. The customized option for Ok button include 'Ok to cancel AF', 'Ok to center', 'Ok to enable AF'. I used to have 'Ok to center' as it easily helps me to move the focal point back to center if I move the focal point. Now I pick 'Ok to cancel AF'. When I don't have the focus lock, I click both the 'OK' and shutter together. Not as convenient as the AF.C, but it helps to overwrite the focus lock being disabled in AF.S mode when focus is not confirmed in AF.

Thanks to you for the help and suggestion. I will call B&H and see if I can return the lens. I don't know a store in Bay Area that I can run to to try my k100d body with another da50-200. I love my pentax but I am starting to feel the lemon in my first gear with pentax.

Thanks,
Hin
04-12-2007, 08:58 AM   #7
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I'm about to commit sacrilege here

But I'm not a big fan of this lens. I know that many swear by it, but I've owned two copies of it so far and both have had problems. I could get focus lock on the first copy but it had serious front focus problems, definitely the lens, not the camera as all my other lenses focused fine. I returned it. Like you I have focus lock problems with the second, but since I mostly manually focus anyway that's not a big deal for me. What I do mind is that the lens is decidedly soft at infinity focus, especially at 200mm. I still own it as it is nice and sharp closer in and for most stuff under about 150mm, but many times I leave it at home.

NaCl(not that any of this helps you with your problem, but the DA 50-200 ain't all it's cracked up to be IMHO)H2O
04-12-2007, 10:23 AM   #8
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Quality maybe a problem

Though the problems, I like the lens. What other lens you find better suited for that focal range to compliment the kit lens together for novice like me, with price consideration. The pentax k100d is my 1st dSLR and I am beginner and I may not be able to tell if something is really good or not. I wish I don't have the focus lock issue. With two days of test shots, I like what I see.









I have called up B&H photo and they can't tell if my problem is on the lens or the camera and I opt to return the lens with a RMA for defective lens.

If others also have these problems, please feel free to reply so that I can gather the information to decide if I should order the 2nd copy. It is likely that I may get another lemon. Or maybe this is a problem that should be ignored -- use AF.C, manual focus, 'ok to cancel AF' but that is not quite acceptable to me.

Thanks,
Hin

04-12-2007, 06:54 PM   #9
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QuoteOriginally posted by hinman Quote
If others also have these problems, please feel free to reply so that I can gather the information to decide if I should order the 2nd copy. It is likely that I may get another lemon. Or maybe this is a problem that should be ignored -- use AF.C, manual focus, 'ok to cancel AF' but that is not quite acceptable to me.

Thanks,
Hin

I suggested contacting Pentax, which was apparently ignored advice :-)

My 50-200 works fine and has better IQ than the Sigma 70-300 I am trying to phase out. My informal tests shows it to be about the same IQ as the touted Sigma 18-50, though in actual use that one seems the sharpest in my bag.

Here's the link to my informal shots for giggles.

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/4768-lens-comp...-70-300-a.html
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