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04-20-2012, 11:21 AM   #1
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DA* 16-50 Difficulty Focusing

Hello all,

long time lurker, first time poster. I have just finished shooting an event with my K-5, DA* 50-135 and DA* 16-50. The 50-135 behaved admirably in a rather large and poorly lit room (f2.8, 1/40, ISO 3200) but the 16-50 was having great difficulty in focusing. At 16mm it would almost never focus and at 20mm or above it would only focus once in about ten attempts. Again, the 50-135 had no issues whatsoever. Manually focusing the 16-50 helped me save the day (greatly aided by a KatzEye Plus Split Prism w/Microprism Collar, for which I would like to give a ringing endorsement).

I guess my questions are: does the 16-50 normally behave like this? Is this a sign of the dreaded SDM failure? How can I confirm if it is?

Any suggestions would be greatly appreciated.

Mario

04-20-2012, 12:23 PM   #2
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QuoteOriginally posted by CptVoltarg Quote
I guess my questions are: does the 16-50 normally behave like this? Is this a sign of the dreaded SDM failure? How can I confirm if it is?
It's not about the lens itself, this is an inherited issue with wider focals, hence the focusing difference between the 50~135 and the 16~50. Looking at your "(f2.8, 1/40, ISO 3200)" note, that is pretty poor lighting and with a wider focal length contrast points are spread thin (meaning the appearance of quite a bit smaller, etc) and thus focusing is a little trickier as there is a very very thin contrast point for the AF system to lock on and the slightest movement of the body effects the locking on of that particular contrast point.

To give you an example, take your 50~135 and focus on something at the minimum focusing distance at around 75mm, then take your 16~50 and focus on that same spot at 16mm at about the same distance then compare the two, there is a much bigger contrast point in the 50~135 shot as opposed to the 16~50 shot.
04-20-2012, 12:32 PM   #3
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Thanks Joe. I guess I was comparing it to the DA 15mm Limited and the 17-40 L from my Canon setup (original 5D) which didn't have as difficult a time to focus. I wonder what the reason is in the difference between the 15 Limited and 16-50.
04-21-2012, 05:58 AM   #4
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I had this with my 1st copy of this lens after some months of use without any trouble.
It was eventually replaced by Pentax (after some extremely tiresome experiences with them).
So, it is definitely not normal.

04-21-2012, 07:26 AM   #5
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Thanks Bart. I may take it in for a look.
04-21-2012, 08:37 AM   #6
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The camera defines the focus not the lens - maybe the firmware has an influence, but that again is part of the camera. My 55 mm lens with SDM is also completely off under the conditions you mention - and the camera indicates good focus! I would prefer no focus confirmation at all. Manual override works fine with a decent screen.
Camera movement is not the cause when I say off I mean OFF not by a 1 mm in 5 m, but by 2 m in 5 m - complete non off focus.
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