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08-25-2008, 02:03 PM   #1
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My warranty serviced Tamron 28-75 is back focussing

and the service manager along with the customer service rep that I've been conversing with via e-mail says that the sample images look great!

I had found that my images weren't sharp as the images being posted on this forum at F2.8 so I sent it in for warranty work. It came back worse than it was.

A couple test images shot at that they think look great. What do you think?

1/3200s f/2.8 ISO200 75mm focus point was on the dogs right eye.



1/4000s f/2.8 ISO200 75mm focus point was on the head of the dog.



A few of the focus tests that I showed to the Tamron customer rep.

1/4000s f/2.8 ISO200 28mm


1/4000s f/2.8 ISO200 38mm


1/4000s f/2.8 ISO200 50mm


08-25-2008, 02:06 PM   #2
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What camera body? At this point, I'd suspect that as a candidate for the problem also.
08-25-2008, 02:33 PM   #3
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It's a K100DS. I ran a focus test with my P-FA 50 1.4 for a comparison and they turned out fine.

Here's one at 1/4000s f/2.8 ISO200 50mm taken with the 50 1.4
08-25-2008, 03:37 PM   #4
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M default assumption is that any FF or BF problem is the body, because the way the Pentax focus system works, it would be very difficult to design a lens to do anything other than focus perfectly if the body is calibrated correctly. That is, if the AF sensor really is the same distance form the lens that the imaging sensor is, how could the lens possibly cause the light to focus differently on one than the other? I mean, I'm sure the laws of phsyics permit *some* sort of way for this to be possible, but given how *easy* it for the body to be at fault - a slight misalignment of the AF sensor relative to the imaging sensor - I'd have to wager over 99% of all focusing problems are the fault of the body.

As for the fact that the FA50 seems better, well, I'd note that the 28-75 wasn't too bad at 50mm either. If the AF sensor is misaligned, it really shouldn't be surprising if it affects some focal lengths more than others.

08-25-2008, 04:17 PM   #5
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Here are some tips:

1. You can get the tamron 28-75 to either front focus or back focus slightly depending on where the lens was focused just prior to focusing the most recent focus point. If you first focus an an object closer to the target then focus on the target, the focus will slightly to the front. If you first focus on an object behind the target and then foucs on the target, the focus will be slightly to the back.

2. Use freshly charged batteries and try your focus test again.

3. Do another focus test but at 75mm and F2.8. Repeat the test and see if the tip No. 1 makes any difference depending on focus in front of the target and behind the target just prior to focusing on the main target.

4. If you get the focus tests right at 75mm and F2.8, you will find that at 28mm and F2.8 there will be some backfocusing. This lens can not get both long and short focal distances right. You would have to make some compromises with this lens. My lens slightly front fouses at 75mm and slightly back focuses at 28mm. It is okay in middle focal lengths.
08-25-2008, 05:44 PM   #6
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QuoteOriginally posted by Marc Sabatella Quote
M default assumption is that any FF or BF problem is the body, because the way the Pentax focus system works, it would be very difficult to design a lens to do anything other than focus perfectly if the body is calibrated correctly. That is, if the AF sensor really is the same distance form the lens that the imaging sensor is, how could the lens possibly cause the light to focus differently on one than the other? I mean, I'm sure the laws of phsyics permit *some* sort of way for this to be possible, but given how *easy* it for the body to be at fault - a slight misalignment of the AF sensor relative to the imaging sensor - I'd have to wager over 99% of all focusing problems are the fault of the body.

As for the fact that the FA50 seems better, well, I'd note that the 28-75 wasn't too bad at 50mm either. If the AF sensor is misaligned, it really shouldn't be surprising if it affects some focal lengths more than others.
The Tamron back focusing approximately 8mm at 50 isn't too bad?
If the Tamron is really within specs, how do I go about getting the body repaired under warranty? It still has 2 1/2 months warranty left.
Would Pentax repair the body under warranty, if it does have a faulty auto focus, based on focus tests using a 3rd party lens especially since the focus tests of the FA50 are better?

QuoteOriginally posted by Stratario Quote
Here are some tips:

1. You can get the tamron 28-75 to either front focus or back focus slightly depending on where the lens was focused just prior to focusing the most recent focus point. If you first focus an an object closer to the target then focus on the target, the focus will slightly to the front. If you first focus on an object behind the target and then foucs on the target, the focus will be slightly to the back.

2. Use freshly charged batteries and try your focus test again.

3. Do another focus test but at 75mm and F2.8. Repeat the test and see if the tip No. 1 makes any difference depending on focus in front of the target and behind the target just prior to focusing on the main target.

4. If you get the focus tests right at 75mm and F2.8, you will find that at 28mm and F2.8 there will be some backfocusing. This lens can not get both long and short focal distances right. You would have to make some compromises with this lens. My lens slightly front fouses at 75mm and slightly back focuses at 28mm. It is okay in middle focal lengths.
Thanks Stratario,
I just tried #1 using a couple sets of batteries, one of them a new set of e2 lithiums. Both of the test shots have the same back focus of at least 3+ feet. The subject was about 25 feet from me.

What I find funny is that both the Tamron CS rep and the service manager saying the two photos of the dog in my original post are fine even though the focus points, right eye and head, are clearly not in focus.

I'm wondering now if both the lens and body are at fault.
08-25-2008, 06:00 PM   #7
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Yeah, from those photos, that level of BF is not acceptable. My copy (on K20D) doesn't do that, it's pretty much spot-on from what I can tell...
08-25-2008, 06:04 PM   #8
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Well, I'd say the Tamron reps need to have their glasses adjusted... Clearly the dog is the object of focus and is blurry while the rocks behind him are clear as bell. I don't know enough about the focusing circuitry or mechanism to offer my advice in that area, but i do agree that the pictures do not look right.

08-25-2008, 07:09 PM   #9
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QuoteOriginally posted by jsherman999 Quote
Yeah, from those photos, that level of BF is not acceptable. My copy (on K20D) doesn't do that, it's pretty much spot-on from what I can tell...
QuoteOriginally posted by madmikess Quote
Well, I'd say the Tamron reps need to have their glasses adjusted... Clearly the dog is the object of focus and is blurry while the rocks behind him are clear as bell. I don't know enough about the focusing circuitry or mechanism to offer my advice in that area, but i do agree that the pictures do not look right.
My thoughts exactly. Now the question is, how do I convince the reps that they need glasses or have them adjusted?

Here is the e-mail that I received from the rep. I'm guessing s/he forgot to add "n't" to the word "does" in the second sentence.

"Now those images look great. I had the service manager look them over and he says they are well in specification and if you send in the lens, we will check it again, as will as the test shoot but looking at those images, he does think there can be anymore adjustments done. The macro area of the lens is a capability that the lens has and not it's strength. If we adjust that area of the lens, it may affect the normal shooting area which looks fine.

Tamron USA "
08-25-2008, 08:32 PM   #10
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Maybe you can make it clear to them that the focus was on the dog and not the rocks behind him. Maybe they'll get it then. I don't know. How they can call that acceptable, I don't know.
08-26-2008, 11:23 AM   #11
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QuoteOriginally posted by RollsUp Quote
The Tamron back focusing approximately 8mm at 50 isn't too bad?
I guess I don't see it that way. I see the zone of acceptable focus extending from around 2mm in front of the line to about 10mm in back. It's tough to tell exactly because the chart is not centered and is also rotated a little.

QuoteQuote:
If the Tamron is really within specs, how do I go about getting the body repaired under warranty? It still has 2 1/2 months warranty left.
I assume the first step would be to call the nearest Pentax service department.

QuoteQuote:
Would Pentax repair the body under warranty, if it does have a faulty auto focus, based on focus tests using a 3rd party lens especially since the focus tests of the FA50 are better?
Good question, but one you'd have to ask them.

Anyhow, if I were you, I'd forget the dog pictures as proof of anything whatsoever. You may have intended the dog to be the focus, but you can't prove the camera atually chose to focus on the dog. Maybe it saw some bit of higher contrast behind it and chose to focus there instead. Each focus sensor covers a pretty large area, so you can't entirely trust the much smaller light that shows you where the focus happened. Concentrate on getting good test chart images, because that is the onl;y way to really be sure the camera even *tried* to focus focus where you wanted (because the test is specifically designed to give the AF system no choice).

I'm not saying it is impossible that the lens could be at fault, but I've yet to hear a good scientific explanation of how that would even be possible. The fact that the Tamron people seemed to acknowledge it was *possible* to adjust suggests there is way for that to occur, but I just can't imagine what it would be, whereas it is is obvious how the camera could be at fault, and indeed, it is obvious that it be next to impossible for a camera to nail this *perfectly*. So I'd want to do some much more rigorous testing to rule out the much more likely possibility first.
08-26-2008, 04:37 PM   #12
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I don't know what to make of this lens

I did another focus test, this time with the focus chart reasonably square. Here's what I got.

1/640s f/2.8 ISO200 28mm


1/800s f/2.8 ISO200 55mm


1/640s f/2.8 ISO200 75mm


These 1:1 samples were taken yesterday evening.

The focus point is about 2/3 of the way down from the top of the image on the tire. I moved it to show where the lens is focusing on this image.


Focus point was on the arctic box.


Focus point was on the window below the wire.

All the other focal lengths show the same IQ as the sample images above. A house that is roughly a 100 yds behind the house w/arctic box is clearer (focused?) than the house w/arctic box. But the image of the red building where I focused on the window nothing seems to be clear, not even a rocky hill that is about a 1/2 mile behind the building, nor any other objects or buildings that are closer than the hill behind the red building.

I just shot the same subjects today and they are the same as the ones shot yesterday evening.

The Tamron rep will send me a call tag tomorrow via e-mail. What is a call tag anyway? And I replied with the cropped images attached along with the photo of the dog with the basic exif data.
To give the rep and service manager some credit about there eyes/glasses needing to be checked over, they thought that the dog's head in the first image of my original post was caused by motion blur. The shutter speed was 1/3200 sec for that image.

Any clues on why the focus test at 75mm is fine while the real world images at all focal lengths, including 75mm, are not good at all?

Crops from 75mm




Would these be the symptoms of de-centering? I've heard of it but I have never quite understood what it is.
08-26-2008, 04:58 PM   #13
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Please look at the first three charts you posted in your last post and then re-read what I said in post #5 above. That's just the way this lens behaves.



QuoteOriginally posted by Stratario Quote
4. If you get the focus tests right at 75mm and F2.8, you will find that at 28mm and F2.8 there will be some backfocusing. This lens can not get both long and short focal distances right. You would have to make some compromises with this lens. My lens slightly front fouses at 75mm and slightly back focuses at 28mm. It is okay in middle focal lengths.

Last edited by Stratario; 08-26-2008 at 05:08 PM.
08-27-2008, 04:03 PM   #14
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QuoteOriginally posted by Stratario Quote
Please look at the first three charts you posted in your last post and then re-read what I said in post #5 above. That's just the way this lens behaves.
I did re-read your post after posting the test images and I understand it. What I find puzzling is all my photos taken at 75mm f2.8 are very soft, awful would be a better word to describe them. None of them are anywhere near sharp, even up to f5.6.

I'm sending this lens back to Tamron tomorrow with sample images taken at f2.8, f5.6 and f8 along with the latest focus tests, and I hope that Tamron can adjust it so that I can start taking sharp images with it at f2.8.
06-06-2009, 12:34 AM   #15
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QuoteOriginally posted by Stratario Quote
...
4. If you get the focus tests right at 75mm and F2.8, you will find that at 28mm and F2.8 there will be some backfocusing. This lens can not get both long and short focal distances right. You would have to make some compromises with this lens. My lens slightly front fouses at 75mm and slightly back focuses at 28mm. It is okay in middle focal lengths.
- You are right. I have K20 and the third copy of that lens (1-massive ff, 2-decentering). Has easy FF on 75/2.8 and adjusted to -4 in my AF menu. From that moment ALL pictures at 55-75mm/2.8 at any distance was right on focus and very sharp, like a cheap prime, except human error, of course. But under 43mm started to BF slightly. Worse, it have aperture focus shift at any focal, meaning if I close diaphragm past f4 (f5.6, f8) it has BF roughly 1/10 distance from the AF target (relation with DoF, more preciselly). Even at 75mm/f4 on close headshots portraits with focus on eyes, tip of the nose is not that clear, but ears are good. Tipically, at group shots at ~28-40mm/f6.3 at 5-10meters I must focus on the front ground one step front of first in group and recompose. I forgive it all that quirks because of excelent 2.8 portraits.

Last edited by silverflower; 06-06-2009 at 12:40 AM.
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