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02-21-2011, 04:54 PM   #1
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Unresponsive DA* 16-50 2.8

My DA* 16-50 is behaving strangely. This issue is occurring on my K-7, by the way.

After testing and extensively using my DA 35mm 2.4 AL (which I am in love with FYI) I replaced the lens with my DA*. Trying to focus on something with AF turned on caused my focus indicator light to come on, but the lens focus mechanism did not operate at all.

Of course my first fear was that the SDM motor gave out. About five months ago this very lens was sent off to Arizona because of hesitating focus and a back focusing issue.

After removing and resetting the lens twice and changing the AF/MF switch position several times the auto focus suddenly started working again. This has happened to me twice now. If I put the camera away for a day or so and pick it up again the lens behaves just fine, at least in terms of auto focus (more on that in a moment).

The only item I can think of that is similar between the two occurrences is that I had a traditional, non-SDM motored auto focus lens on the camera before trying to use the DA* 16-50. The first time it was with the DA 35mm AL, the second time it was my Sigma 50mm 2.8 macro.

Another thing I'm noticing is some rather odd softness in the borders and edges using the lens at wide- and mid-point focal lengths. Let's say between 16mm and 35mm. To tell the truth I'm not blown away by the 50mm performance, either.

I might shoot at f/8 or f/11, which is supposed to be this lens' prime performance region for edge-to-edge sharpness, and still see a stark difference between the center and edges.

I'm resigned to the fact that this lens has it's quirks, especially with strong CA at all focal lengths. But can anybody account for the weird auto focus behavior? And as for the softness in the edges, I expected this at f/2.8 and f/4 but not at f/11.

Come to think of it, I seem to recall having very nice edge performance at f/11 or f/13 when I first got the lens 1.5 years ago. I have not been rough with my equipment, but can something happen to the lens to have this kind of effect?

I need suggestions as it seems this lens is going to take another trip before the 6 month warranty runs out on March 16!

02-21-2011, 06:19 PM   #2
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People need samples, or not much help will come :-)
02-21-2011, 06:44 PM   #3
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If what you're saying is that the AF is again temperamental, hesitating or requiring multiple 'goes' to wake the SDM up, then I wouldn't balk at the thought of sending it back for repair.

It's a great lens, and a good copy is absolutely awesome optically, but the AF issues are not rare enough to be a concern.

Get it sorted before warranty expires, and consider longer term warranty on the lens.
02-21-2011, 06:48 PM   #4
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I can account for the autofocus issue. I had the same issue with my DA* 60-250 a few months back and was afraid I would have to send it in for repair just weeks after getting my K-7's level adjusted.

From what I can gather it comes down to this...the lenses with SDM and the quick shift focus system all have a couple of clutches that can disengage each other. When you have the autofocus turned on and manually turn the focus ring, the manual focus clutch disengages the autofocus.

I think there's a similar clutch between the mechanical "screw drive" autofocus and the SDM electronic autofocus. My "theory" (since I haven't physically torn apart either of my DA* lenses to check) is that the mechanical AF will use a clutch to disengage the SDM motor to keep from harming the motor itself.

When I was experiencing the issue I noticed that the manual focus could only turn the focus if I pressed the lens release button which also retracts the mechanical AF "screwdriver". I then took a small screwdriver (about the size of the AF head on the camera body) and gave the attachment on the lens a 1/4 turn. Ever since it has worked just fine.

What I think was happening is that the mechanical AF on the camera was only slightly engaging the mechanical AF on the lens, and ever so slightly pressing on the clutch mechanism.

When I have experienced this issue (and it has happened twice now) I have noticed that the camera would let me focus closer to me and not further away. It would only turn one direction.

I'd be very interested to see if this is the same issue you've been having and also how widespread it is. If it's very widespread at all I think the fix for the issue needs to become a sticky on the forums (and hopefully Pentax will issue some kind of statement about it...doubtful).

Good luck.

02-21-2011, 07:46 PM   #5
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QuoteOriginally posted by SpecialK Quote
People need samples, or not much help will come :-)
Well I suppose the main issue is the AF, for which I really can't show a sample. The auto focus action is 75% of my worries, and if exploratory surgery is the remedy then they might as well address the softness, as well.

I was curious about the unresponsive auto focus because it only seems to occur after having used non-SDM lenses. It's like the camera forgot how to use the DA* 16-50mm. But I know it's not an issue of the contacts being dirty or anything like that; I tested the same situation with my DA* 50-135mm and it works like a champ. Using the 16-50mm it's as if the lens wants to focus but the mechanism simply doesn't operate. Once I get it responding again it moves very slowly at first. Then with repeated action the lens gets faster re-acquiring focus.
02-21-2011, 07:52 PM   #6
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I had similar AF issues with my copy of the DA* 50-135. The problem started after I didn't use it for about a month and stored it vertically. I read in another post about storing it horizontally so I tried it. Now I only store, on or off camera, horizontally and try to use the DA* frequently. Haven't had the AF lock up now for about a year. It will still occasionally lose focus in low light but will recover without turning off the camera.
02-21-2011, 08:07 PM   #7
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I store my DA* lenses vertically in a camera bag......would you mind passing on a link to the other post about this being a potential issue?

Also what is the deal with DA* lenses needing regular workouts....any solid hypotheses about what happens if the AF is not engaged for a few weeks or a month while in storage?
02-21-2011, 08:12 PM   #8
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There is supposedly some capacitor in SDM lenses that holds a charge for only weeks before going flat. Once flat it can take a while being mounted on a camera before it is charged enough for the lens to work.

This is all conjecture, whether there's any truth in it is anyones guess.

Certainly this is the first I've read how storage position can affect the lens. That sounds like bunk to me, but then the tempremental nature of SDM is enough to make even the hardened sceptic very superstious

02-22-2011, 05:05 AM   #9
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02-22-2011, 07:05 AM   #10
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