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12-17-2016, 01:53 AM   #1
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HD Pentax-DA 18-50mm F4-5.6 DC WR RE

My daughter complained about slow, bad focus of her DA18-50 on her K-S2. I tried it on my K-1, thinking it might be camera related. The lens is very slow, hunts and misses focus often.

I'm taking it back as it's 8 months old and did work well earlier.

I know the DA*16-50 suffers from SDM failure but thought the DC motor was reliable.

Any other failure experience with this lens?

12-17-2016, 02:14 AM   #2
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I'd send it in- maybe it needs calibration. Is the manual focus OK? Since this lenses focuses by wire, if the MF isn't smooth then that would explain the AF issues.

Can't say I've seen a report of one of these failing, though.
12-17-2016, 03:13 AM   #3
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QuoteOriginally posted by Timd Quote
I'm taking it back as it's 8 months old and did work well earlier.
Yes, do it. Though, first I would advise you check the following:
- make sure the lens mount and the camera mount are clean, free of particles, grime, or even damage
- make sure the front and especially the back of the lens don't have smudges or dirt. If you decide to clean the lens, read some tutorials first. But sometimes something like a fingerprint on the back lens element can cause odd effects
- make sure the lens locks into place correctly when you mount it
- make sure you are using the right AF modes. If you are using Auto 11, then the camera might be working as intended, but simply locking focus in places that you don't expect, for example. AF.C is usually slower than AF.S.
- make sure you don't have any special AF options enabled, like focusing fine adjustment (focus correction)
- make sure you don't have a filter mounted on the lens (some filters can mess with AF, particularly old polarizing filters)
- do a test in good, bright daylight. Indoors our eyes often give us the impression there is plenty of light, but a lens with that aperture will not get enough for reliable AF.

I don't think we had many complaints about this lens yet. The 16-50mm f2.8 would be better in low light, because it has much faster aperture. But yes, the SDM has a bad reputation and the DC motor has a good one, so far. However, the problem you described seems to have more to do with locking focus, not so much with the motor itself.
12-17-2016, 08:09 AM   #4
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I know Pentax lenses aren't quick, but this is glacial. Like 3 seconds to move from near focus to far focus.

It's not back or front focus issues, it's so slow that the camera gives up.

I'll clean the mount. But I don't hold hope.

12-17-2016, 10:24 AM   #5
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Out of curiosity I pulled my 18-50 out and its maybe a half second from close to infinity. Hope it is contacts.
12-17-2016, 01:00 PM   #6
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As Adam pointed out, the manual focus is by wire so it needs the focus motor. Playing around it is not very slow with manual focus. Hence the motor works, but as on Auto focus, it moves rapidly in fits and starts. It rarely gets focus as it overshoots and gives up. I've cleaned the contacts but it still doesn't work.

I'll let you know when I get it seen to.
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