[vanyagor]

I did the search and I've read a bunch of threads related to the issue. However I have not seen this particular one addressed.

I have a new K5 + Tamron 17-50mm f/2.8. I also have a Pentax 15mm f/4.0 Ltd lens.

The Pentax lens is spot on, no AF adjustment needed.

The Tamron at 50mm requires (at least) -10 adjustment (it is front focusing),
at 17mm it is however noticeably back focusing. Adjustment +3 seams to fix that.

As far as I understand it is correct that AF is different at different focal lengths. So, can a zoom lens be optimized for AF at all focal distances at all? In my case the range from -10 at the long end to +3 at the wide end seems too much, kind of kills the whole point of AF fine tuning.

Your input is valuable! I need to decide what to do with the lens. Other than this back/front focusing issue I really like it.

If you own the lens, could you please post your data for wide/long end focusing?

Thanks

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Last edited by vanyagor; 12-15-2011 at 01:40 PM.

[Adam]

Those seem like somewhat excessive adjustments, so I would exchange the lens or send it in for adjustment.

[jane7l]

send it in for adjusting. usually no problem solving this problem. had mine sent in, works flawlessly now.
good luck

[vanyagor]

Thanks Adam and Jane. I guess this is what I will do even though I don't want to part with the lens even for a couple of days This kind of adjustment should be covered by their warranty, right? And I have to contact Tamron directly, correct?

Thanks for your input.

I'm still interested if other people have experienced same behavior (back focus at one focal length and front focus at another). Jane, is that what you had?

[audiobomber]

I've read about this a number of times with the Tamron 17-50. I'm surprised you didn't see it in your research. AF corrections in the camera won't help. Send the lens back, it's defective.

[mr tibbs]

I had to send my first copy of this lens back. The second one came spot on! It's been a great lens for me so far.

[vanyagor]

I've read about this a number of times with the Tamron 17-50. I'm surprised you didn't see it in your research. AF corrections in the camera won't help. Send the lens back, it's defective.

Thanks Dan. Neither of the threads I saw discussed both back focus and front focus for the same lens at different focal distances, but then again, I'm maybe not that good at using the search. If you see some relevant threads, please post a link here.

[darrenleow]

My 17-50 front focused badly at 17mm, so I had to zoom in to 50mm, focus and zoom back out. I thought it might have been Pentax's AF system performing badly at the wide focal length but I just sent it in to Tamron hoping that it will get fixed.

[K6-III]

Tamron's service department is pretty spectacular, so I'd just send it in.

[darrenleow]

An update:

Repairs: 002 TAMRON
F FOCUS DEFECT OR POOR IMAGE QUALITY
1 GENERAL CHECK, CLEAN, AND ADJUST
2 TURNAROUND TIME APPROX 5 BUSINESS DAYS 800-827-8880

I hope when it comes back it'll be working fine

[FruitLooPs]

Mine is the same, however I got it from the marketplace I might just leave it be. Very nearly passable in my case, focus charts up close show after best adjustment 17mm just barley in focus with a fair bit of backfocus bias, same at 35mm and spot on balanced at 50mm. If i want wider focal lenghts balanced then 50mm is too far out of focus.

[vanyagor]

An update:

Repairs: 002 TAMRON
F FOCUS DEFECT OR POOR IMAGE QUALITY
1 GENERAL CHECK, CLEAN, AND ADJUST
2 TURNAROUND TIME APPROX 5 BUSINESS DAYS 800-827-8880

I hope when it comes back it'll be working fine

darrenleow, have you received the lens back? is it working as it should now?

I have mailed mine for service last Friday and I have received the following repair acknowledgement:

Repairs: 001 TAMRON
F FOCUS DEFECT OR POOR IMAGE QUALITY
AF AUTO FOCUS IS NOT FUNCTIONING CORRECTLY
1 GENERAL CHECK, CLEAN, AND ADJUST
2 TURNAROUND TIME APPROX 5 BUSINESS DAYS 800-827-8880

Now waiting for them to ship it back. I will report when I receive the lens back.

[vanyagor]

Got my lens back. Quick (unprecize test) shows the lens is now front focusing on both ends - wide and tele. Adjustment "-10" comes just short of fixing it I will do tests more carefully a bit later. Another problem I have noticed: I get consistently underexposed shots by 1 1/3 stop (tungsten light). Will do some more research/testing before jumping to conclusions. Please let me know if it is a known issue.

[chickenandavocado]

i think under exposing at least 1/3 EV is normal for this lens, someone put up a comparison somewhere proving this and this also seems to be my experience too, just need to increase 1/3 EV and all is good.

[vanyagor]

i think under exposing at least 1/3 EV is normal for this lens, someone put up a comparison somewhere proving this and this also seems to be my experience too, just need to increase 1/3 EV and all is good.

I my case it was 1+ 1/3 = 4/3 stops and that is a bit too much. Still need to test it under various lighting conditions