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01-03-2010, 12:29 PM   #1
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Uh oh, I think my 17-70mm SDM is having problems

Hi all,

I really didn't think my SDM lens would fall victim to any issues, but it seems that I may indeed have a problem. I've had the lens for about 9 months now, so luckily the company I've bought it from will get it to Pentax for repair or replace.

The focus mechanism hasn't failed and does indeed work... sort of. My problem is that between 17mm and around 35mm, the focus mechanism (particularly obvious in low light) seems to "creep" in "steps" instead of moving in one swift motion. This is accompanied by a clicking sound as the motor inside the lens engages and disengages. The lens / camera focus should sort of "swooooooosh, beep" but my lens goes "click, click, click, click, click, click" (small movements of the focus ring) and eventually it locks focus. On the odd occasion, it has also simply refused to lock focus at 17mm even in the brightest sunlight. Weirdly, I've had no problems between around 50mm to 70mm, infact the lens is very snappy indeed.

Pain in the bum, but i'll let you know how it goes.

PS on a K20D, also tried to clean the contacts to no avail.

01-03-2010, 02:22 PM   #2
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Sorry Big G, this is normal behaviour.
All SDMs do this in low light conditions.
It's not a sign that the SDM is failing, it's simply an inability to lock focus under the low lights, hunting and rechecking itself before committing to the focus point.
As you've found also, even in good conditions the SDM AF fails to lock focus, and for me it's at infinity focus on my 16-50. All these little nuances of the SDM technology - I really hope Pentax listens to our concerns and develops a newer generation of tougher, reliable and faster SDM.
01-03-2010, 02:30 PM   #3
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Oh no Ash, that's really not good news!

However I get the same clicking issue at the wider angles under bright sunlight conditions, it just happens to be almost useless in low light conditions as it's far too slow.

By this checking and rechecking, does the "click, click, click" around 15 to 16 times sound normal? Do your SDM lenses do the same? Bear in mind mine will do the same in low and bright light, just that bit faster in bright light.
01-03-2010, 02:46 PM   #4
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No, it shouldn't be hunting in good light - that's not normal.
But, depending on how still the camera and lens are when trying to focus, in low light it is known to keep trying to focus in and out (with that faint click, click sound) until it's able to settle on the right focus point.
Basically the subject needs to be perfectly still for a few seconds to ensure that it hits the mark.
Being f/4 it will struggle some more in low light, but I don't get why it has to hunt in bright conditions. If this behaviour is new, best get it checked out.

01-03-2010, 04:18 PM   #5
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Yeah it's the bright light hunting that I suspect is an issue, I could live with the low light hunting but when it takes around 15 "clicks" or adjustments of autofocus before locking on, there must be something wrong with it. As I say, 50mm to 70mm is very fast and snappy.
01-03-2010, 08:42 PM   #6
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QuoteOriginally posted by Big G Quote
Yeah it's the bright light hunting that I suspect is an issue, I could live with the low light hunting but when it takes around 15 "clicks" or adjustments of autofocus before locking on, there must be something wrong with it. As I say, 50mm to 70mm is very fast and snappy.
Does it slowly creep in one direction? Or does it move back and forth? If it's slowly creeping only in one direction, I'd say it's going to die soon. If it's re-adjusting in the normal way, I have no idea what the problem is.

EDIT: I'd start looking for a Pentax repair centre.

Last edited by asdf; 01-03-2010 at 10:59 PM.
01-04-2010, 02:16 AM   #7
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QuoteOriginally posted by Big G Quote
(particularly obvious in low light) seems to "creep" in "steps" instead of moving in one swift motion.
My 17-70 does this from day 1. It locks focus with a few (something between 2-5) "clicks". I noticed that there are more "clicks" in low light and/or if the focus throw is minimal (i.e. there are more "clicks" for focusing from 2m to 3m than focusing from 1m to 5m).

QuoteOriginally posted by Big G Quote
The lens / camera focus should sort of "swooooooosh, beep"
Mine almost never did that. It does a long "swoooosh" only if it has to turn the focus ring from one extreme to the other (i.e from infinity to 1m distance or the other way around) and after that it does a few "clicks", so it is like "swoooooosh, click, click".

Mine is in its 11th month, no issues with focusing so far (knocks on wood)
01-04-2010, 02:56 AM   #8
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QuoteOriginally posted by asdf Quote
Does it slowly creep in one direction? Or does it move back and forth? If it's slowly creeping only in one direction, I'd say it's going to die soon. If it's re-adjusting in the normal way, I have no idea what the problem is.

EDIT: I'd start looking for a Pentax repair centre.
It doesn't hunt back and forth, it only does it in one direction. I will try to make a video of it today and post it for you all to see.

01-05-2010, 01:55 AM   #9
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I was also thinking about making a video but it was too late in the evening.
One thing to consider: 17-70 seems to focus a bit different than 16-50 (and probably 50-135). I checked it with fellow photographers having 16-50 and 17-70. The 16-50 lenses seemed to do less "clicks" than the 17-70 lenses on same bodies, focusing on same scene.
There are also differences between the hardware of 17-70 and 16-50: the former is KAF3 and the focusing ring turns during AF operation, while the latter is KAF2 and the focusing ring doesn't turn during AF operation. The internal mechanical differences can also be related to the slight differences of autofocusing.

By the way, Pentax SDM lenses doesn't really "swooooosh, beep", that's Cannon and Sigma HSM which does that
[YT]http://www.youtube.com/watch?v=ngUcQUchHEU[/YT]

Last edited by simico; 01-05-2010 at 02:00 AM.
01-06-2010, 01:15 PM   #10
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Here's how mine works in moderate/lowish light. You can hear the "clicks" (turn up the volume).
[YT]http://www.youtube.com/watch?v=Ewcgq9tmPfw[/YT]

Ps: I have a crappy videocam, so quality is...
01-06-2010, 01:28 PM   #11
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Simico, that's pretty quick in low light compared to mine, I will try to make a video this evening or tomorrow - need to dig out my N95 phone though.
01-06-2010, 01:50 PM   #12
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QuoteOriginally posted by Big G Quote
Simico, that's pretty quick in low light compared to mine, I will try to make a video this evening or tomorrow - need to dig out my N95 phone though.
Yours must be really slow then. The focusing looks slow and quite inconsistent to me. Do you know what the LV was for that test simico?
01-06-2010, 02:01 PM   #13
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QuoteOriginally posted by PentaxPoke Quote
Yours must be really slow then. The focusing looks slow and quite inconsistent to me. Do you know what the LV was for that test simico?
You wouldn't believe how slow it is at 17mm. Really, I think you guys will agree something is amiss .

Edit: i've got my videos up on Vimeo, they will be ready in an hour (haven't paid for an account yet, unsure how much i'll use it).
01-06-2010, 02:10 PM   #14
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Depends on how 'low' you can go with the ambient light.
With a still subject and ordinary night lighting in the house, the focusing shouldn't be too much slower than simico's example.
01-06-2010, 02:22 PM   #15
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Ok gents, what do you think of these. The lighting appears brighter in the video than it actually is. Distance to subject was around 6ft, ISO 100, white balance set to tungsten, f/4.

Apologies for the out of focus video, I had to use my girlfriend's Sony Ericsson phone to make the vids, but at least it gives you an idea of what I'm dealing with.

17mm:



70mm:

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