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07-18-2016, 06:58 PM   #1
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K3 and tamron 70-200

Hi Guys

I posted a thread a good while ago about the K3 and my Tamron 70-200 having issues focusing in Live view at distance. I don't use live view much but it effects video too.

the subject is quite badly out of focus and I have done some more investigation into this.

What I have found is that there is about a 1/4 turn backlash on the lens screw drive - when using normal phase detect the hunting seems to cope with this although may explain the slight oof images i get sometimes - when using live view / video the slow contrast detection seems to rack all the way back and then in again and then stop - but always stops out of focus. If i have the lens focus on MF (is engage the MF ring on the lens) and give it a very slight twist - it brings it back in focus properly. I think the very short throw this lens has is causing the issue when combined with the backlash.

Has anyone got a 70-200 they can check with this please - and also check the backlash on the screw thread - i just used a small flat screw driver and observed how much of a turn there was with no resistance.

Thanks in advance

Steve

07-18-2016, 07:12 PM   #2
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I don't have the 70-200mm any more, but mine worked fine in live view with the K-30. Sounds like a defect to me.

Or it could be an incompatibility with the K-3's more powerful/violent AF motor.

Adam
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07-18-2016, 07:56 PM   #3
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My tammy doesn't focus in LV on my K-5 at all. It just never finds focus lock. However with viewfinder and PD it focuses well (relatively quick and accurate) - although it is most accurate 100 - 200mm with slight back focus as 70mm.
I know of at least 2 others that have the same problem with LV - in fact there are few in the user reviews for this lens.

My screw drive has about 1/8 of a turn play in it, so sounds like probably less than yours. Not that it helps mine focus in LV.

Anyway it does stop me using it, as I mostly use the viewfinder and can always use MF if I'm in LV anyway.
07-19-2016, 01:06 AM   #4
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Thanks Adam and Kiwi

Be interesting to hear from more users - it sounds like the backlash combined with the very fast focus throw of the 70-200 is what causes it. From a bit more testing I am figuring that the following happens

Focus is initiate > Finds focal point but carries on driving focus all the way back > once stop is reached it focused back to its first found point - due to the backlash the point of focus is actually another 1/4 turn of the screw in this direction therefore the focus is out. Manually nudging it with the focus ring on the lens takes up the slack and brings it back into focus.

It looks like either the tamron is not compatible (and as its still being sold probably still isnt), I have excessive wear in the gearbox (there are only 3 gears) or the drive shaft / mechanism or the focus algorithm in the camera is wrong for this lens - its also very slow to focus compared to the one pentax lens i have! - PDAF seems fast and also throws the focus back and forth a lot while getting the focus right - which probably takes up the backlash enough.

If anyone else can chime in with their findings i'd greatly appreciate it. It looks like I may be saving up for some non screw drive lens's which leaves me with Pentax ($$) or second hand sigma - which are hit and miss in the 70-200 range

Steve


Last edited by 5teve; 07-19-2016 at 01:18 AM.
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