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08-11-2009, 06:07 AM   #1
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SDM issue - what am I doing wrong?

I realise auto-focus isn't perfect and in low contrast situations it won't always be able to get a fix. I'm perfectly happy to switch to MF in most situations.

I had a K10D, originally with the kit 18-55 which I later "replaced" with the DA* 16-50. I just replaced the K10D with a K-7 (trading in the K10D and 18-55). I have just received the DA* 50-135 - really just received, so I haven't tried it yet.

The K-7 18-55 combination (i.e. not SDM) hunts when it has difficulty getting focus. Sometimes the hunt won't get a lock, but other times it will. Using the 16-50 (SDM) with either the K10D or the K-7 in single focus mode it sometimes just sits there doing nothing when the shutter is half pressed. When there's no contrast at the focus point I can understand this, and sometimes moving to focus on a hard edge does the trick (and that's fine), but other times it refuses to move, wherever I point it.

I have two workarounds: the first is to use the focusing ring to defocus the lens (e.g. set it to closest focus); the second is to put my hand right in front of the lens and get it to focus on that (it usually works). Then I return to what I was trying to focus on and it usually locks in.

The workarounds aren't perfect and often it would be quicker and easier to focus by hand (except that I would also need to select MF), but that isn't what annoys me. Its mainly the fact that the lens will sit slightly out of focus with the focus light flashing and refusing to move.

Is this normal, am I doing something wrong, or is it my 16-50 (I may exclude that when I try the new 50-135)?

Thanks

Simon

08-11-2009, 07:45 AM   #2
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Have you tried testing different lenses under the same conditions, focusing on the same subject? I think that would be the only to determine if the problem is limited to a specific lens or specific type (SDM) of lens. I really think it's more of a body issue than an issue with the lenses. At least the K-7 has the focus assist lamp to help it along.
08-11-2009, 07:52 AM   #3
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My 50-135 was exhibiting similar behavior, where at times it would simply refuse to adjust focus.
At the times it seemed "stuck" I would focus to infinity and than try again to focus.

I recently sent (Last Wednesday) the lens to Pentax Canada and was told that if it was a big problem it would need to be sent to Japan.
However it is under warrantee and I was told today I should have it (or possibly replacement I would assume) 10-14 days from when I dropped it off.

Long and short of it is that it sounds like your 16-50mm is experiencing similar SDM problems that my 50-135mm did.
My SDM started out like yours and deteriorated to the point it would intermittedly be totally unresponsive.
If under warrantee I would look into having it sent in.
My 2 cents!
08-11-2009, 08:01 AM   #4
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My 16-50mm does the same thing. This erratic behavior has been reported by a number of users. In my case it works well in normal daylight 98% of the time. In lower contrasting light (heavy cloud) it can have trouble locking in. The strange thing is that it works great in low light. I've had the thing lock focus just fine in the worst light but not in somewhat average conditions.

I've just learned to live with it and usually use the quick shift for a second then it will work.

This has been the case on 3 different bodies and my other lenses all work fine in the same situations. Btw, the 16-50 does not have this problem on the *istd which is only screw drive.

08-11-2009, 08:09 AM   #5
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QuoteOriginally posted by Simon Quote
I realise auto-focus isn't perfect and in low contrast situations it won't always be able to get a fix. I'm perfectly happy to switch to MF in most situations.

I had a K10D, originally with the kit 18-55 which I later "replaced" with the DA* 16-50. I just replaced the K10D with a K-7 (trading in the K10D and 18-55). I have just received the DA* 50-135 - really just received, so I haven't tried it yet.

The K-7 18-55 combination (i.e. not SDM) hunts when it has difficulty getting focus. Sometimes the hunt won't get a lock, but other times it will. Using the 16-50 (SDM) with either the K10D or the K-7 in single focus mode it sometimes just sits there doing nothing when the shutter is half pressed. When there's no contrast at the focus point I can understand this, and sometimes moving to focus on a hard edge does the trick (and that's fine), but other times it refuses to move, wherever I point it.

I have two workarounds: the first is to use the focusing ring to defocus the lens (e.g. set it to closest focus); the second is to put my hand right in front of the lens and get it to focus on that (it usually works). Then I return to what I was trying to focus on and it usually locks in.

The workarounds aren't perfect and often it would be quicker and easier to focus by hand (except that I would also need to select MF), but that isn't what annoys me. Its mainly the fact that the lens will sit slightly out of focus with the focus light flashing and refusing to move.

Is this normal, am I doing something wrong, or is it my 16-50 (I may exclude that when I try the new 50-135)?

Thanks

Simon
I don't know if it is normal or not actually, but I do have a similar problem. When in low light, it usually hunts around and then decides it can't focus and randomly goes to the closest or infinity focusing. Then if I try to focus again, it won't budge unless I AF on a lamp or really bright object or I twist the focus ring a little so that it isn't in one of the extremes and it will work again. This usually occurs with all my lenses, now that I think of it (one of them the 50-135mm).

BTW, since I already posted here, I was wondering about my SDM. Is it normal for it to make squeaks when the AF hunts? It doesn't bother me, but it seems really odd. Any how, it doesn't occur outside in daylight, but occasionally in doors when it hunts, it will make a tiny squeak (the SDM motor obviously) every time it changes focusing direction. Just a quick question. I don't really think it is worth getting serviced over (maybe when the warranty is almost over I might get it serviced but that's more b/c of the water that entered the lens).
08-11-2009, 08:31 AM   #6
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Different bodies

Thanks to all so far. I will certainly have to see whether my new 50-135 is better than the 16-50 in this respect. If it is then I know what to pursue.

@maxwell1295 - until I bought my new body at the weekend, I had only tried the 16-50 on the K10D. Since I traded it in I can't put the lens back on that body. When I first used the K-7 I thought the problem was over for a while, then I found a situation where it happened all again. So, yes, I have tried it on two different bodies with similar results.
08-11-2009, 08:37 AM   #7
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it's the dreaded "Sometimes Doesn't Move" syndrome
it's been reported from various users. i don't know how widespread the problem is, but it's really got me concerned about the 16-50.
i've plans to purchase one this year after giving pentax 2 years to iron out the kinks.
but so far, it seems like there are still issues with this lens.
08-11-2009, 09:08 AM   #8
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Don't worry too much

@opiedog - my 16-50 isn't new (about a year old IIRC), so it's possible that if it is a problem it doesn't exist in new lenses. In any case I'd class it more as a minor annoyance than a serious problem. It only becomes anything like serious if I'm shooting action, which isn't all that frequent. YMMV

08-11-2009, 10:29 AM   #9
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My DA* 300mm has "got stuck" a couple of times, both times it was very cold weather, it hasn't done it in warm weather, could it be something to do with the tolerances on the lenses?
08-11-2009, 11:38 AM   #10
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My first copy started acting like that after some 3 months.
It was eventually replaced by Pentax.
The new lens has behaved perfectly since March (so far).
08-11-2009, 11:52 AM   #11
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what's got me spooked is that your issue started happening at about 1 year and pentax's warranty is only 1 year for their pro lenses.
sigma has 4 and tamron has 6.
for pro lenses, it would have been nice to get a longer warranty.... just in case

i still would prefer the 16-50 over tamron's 17-50 or sigma's 18-50 though

Last edited by opiedog; 08-11-2009 at 11:59 AM.
08-12-2009, 03:11 AM   #12
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Sorry if I misled you

@opiedog - No, the problem didn't start at one year. It has always done it. I just didn't think of it as a major problem. I thought the K-7 SDM support might be a little more advanced than the K10D which was a retrofit. I had a small hope that things might get better but it appears they haven't, so I thought I'd just ask the question to see whether it's a universal design issue or a problem on some examples. I'm still happy to live with it - the lens is otherwise fine. I am not sure it's a mechanical "sticking" problem, I suspect a belief by some part of the system that it's either in focus or unfocusable when it's not.
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