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01-07-2013, 03:59 AM   #1
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Pentax K5 Auto Focus Problem

When using my K5 with a Pentax fixed lense the auto focus works fine.
Using my DA* 50-135 lense, it's haphazard, sometimes it finds the "sweet spot" other times nothing happens.
It's the same when I swap over to my K7.
I have the lense on "auto" and also the camera.
I often find myself just using manual, but I thought auto focus should be simple.
Some help will be appreciated.

01-07-2013, 04:44 AM   #2
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send your K5 and 50-135 in for calibration.
01-07-2013, 05:16 AM   #3
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Does 'sweet spot' mean that the AF works OK? If it's only sometimes that the AF is working there may be SDM motor fatigue setting in and worth having it fixed if still under warranty. Is the AF engaging each time? If so, then are you trying to focus on something that has no contrast and this too difficult for the AF mechanism to fix its focus on. If all of that is OK, then it is worth having it looked at by an expert.
01-07-2013, 06:27 AM   #4
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I used to have 50-135 but had lots of problems. It was sent twice for the calibration and after second one I have returned the lens. The lens was of a very good quality in terms of built, colours or contrast but mechanically there was something wrong. I do not know if this was the SMD or it was just assembled not well. Perhaps I got the bad copy Indeed I had AF problems and also you could see that on the left side the DoF was narrower than on the right so I could not accept a such fault on a lens worth 700.

01-07-2013, 07:35 AM   #5
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If you have the same problem on both the K5 and K7, then it's the lens, but, what do you mean by 'sweet spot'? Sharp focus?
01-08-2013, 11:30 AM   #6
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sounds like the SMD motor to me......ive had the same issue twice on my 50-135 - once covered under warranty and it then started again recently. i would often have to try and focus 10+ times before it suddenly started working again and then if i didnt use it for a few days i would have the same problem again.

so i followed the thread listed here and converted mine to use the screw drive focus - now it works reliably all the time
01-12-2013, 09:29 AM   #7
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QuoteOriginally posted by Dobson Quote
I often find myself just using manual, but I thought auto focus should be simple.
Lots of good advice above but lets not assume a problem where one may not exist.

If you are used to manual focus then adjusting to AF can take a little time. First the camera (depending on how you have it setup) can use a number of different 'focus points' or sensors. The camera tries to determine what should be in focus based on an algorithm that includes a number of things including contrast. In short, the camera will try to focus on what it THINKS you want to have in focus. But of course that is not always the case. Also, because contrast is what the camera uses to determine focus if your subject is very low contrast, a white wall, or blue sky with no clouds, then it will have a very difficult time focusing properly.

I have my camera set to 'center point' focus so I know exactly where the camera is going to focus, this essentially turns off all but the center AF sensors. You then must focus on which part of the subject you want to be in focus and then recompose the shot. This is not without some risk as when you recompose you may be moving the camera enough to take your intended point of focus out of focus, depending on your depth of focus with that lens and aperture. It works for me, but there are other ways to get the job done. You can also use focus point select to pick which of the 11 sensors you want to use or use manual focus.

And as noted above, it could be a problem with the lens.
01-14-2013, 04:46 AM   #8
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Auto Focus Thanks for your advice.

Thank you all for replying with your advice, I think it could be the contrast, so I'll carry on practising and see what happens.


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