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07-13-2011, 02:10 PM   #16
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I'd challenge any local shop to even expalin what could possibly be misadjusted in a lens that would cause misfocus. Focusing is done by the camera, not the lens. There are a couple of specific ways that people have proposed over the years where a lens could possible contribute to a problem, and in cases where either of those turn out to be true, it would be possible to figure out which was the case by the specific results of the test. But I have virtually never seen any test posted that actually demonstrates either of these theoretical possibilities. Not saying they never happen, but the evidence is, 99% of the real focus problems discovered are in the camera, not the lens. And 99% of reported problems turn out to be testing error.

In your case, since you said it's mostly being tested in tungsten light, and you're presumably using a test that measures focus at the edges, I still say it sounds like your lens is functioning correctly. It exhibits field curvature just as it is designed to, causing you to perceive FF even in daylight, and on top of that the camera front focuses a bit in yellow light just as it was designed to. Of course, by designed to, I don't mean these were done on purpose. I jusy mean, they weren't specifically designed not to do those things. Generally speaking, only macro lenses are designed to reduce field curvature, and only high end bodies are designed to compensate for the natural tendency of phase detect focus systems to FF in yellow light.

07-13-2011, 08:32 PM   #17
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You're wrong, I've tested with the chart centered, and at the edge so there is no field curvature to deal with. I've tested in natural and in tungsten light. Under the same test conditions, my 55-300 and my tokina 35-70mm, the two lenses I *always* get sharp shots with under all conditions, don't front focus one iota. As I tested more I realized most of my other lenses are within tolerance.. I actually set up a tripod, measured the angles, and did 20 shots for each lens shooting at the center of the chart, and 20 shots for each lens shooting to the edge of the chart. My methodology was to switch to manual focus, focus to infinity, switch to AF, focus and shoot. I set up a tripod and stopped doing it handhold, and after repeating the tests over and over again, my other lenses aren't front focusing much, as I repeated the tests (in daylight thank you) my other lenses (Except the POS sigma) were either spot on or maybe 2mm front focused. However, repeated testing showed the tamron was worse than I initially thought. It has between 6 and 8mm of front focus at the telephoto end, 3 or 4mm at the wide end.

Right now I've got it adjusted -150 microfocus adjustment. That gives me considerably better results across the board. It makes the wide end back focus and the tele still front focuses significantly, but this way my target is usually just barely in the focal plane. So I aim for eyes or ears at tele and noses at wide and seem to get sharper pictures all around. It's just a HUGE hassle to have to change the adjustment every time I swap lenses.

Tamron indicated they could fix it for $140, and their focusing problems are well documented on google, so I don't see how you can say the problem is the camera body.

Charles.
07-13-2011, 10:42 PM   #18
Brooke Meyer
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Some Tamron 28-75 examples over 4 years

From K10, K20 & K5. Lots of airplanes & trips. Lens has been to China 4 times, Japan twice, Vancouver Island twice, Paris, London and Glasgow once. Daily to office in for a year and 20 or 30k dance exposures. Normally, I shoot with two bodies, either 12-24 & 28-75 or 28-75 and 50-135.

Marc is correct. The focus movement in the lens stops when the screw drive stops because the camera says it's focused.

I have 3 lenses that overlap a bit so if one dies, I'm still shooting. If I could only have one, the 28-75 with 12 in min. focus is the most versatile. I've never adjusted a thing on any body. I have made a lot of bad pictures and I was the reason.

Last edited by Brooke Meyer; 08-31-2011 at 10:59 PM.
07-14-2011, 10:09 AM   #19
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QuoteOriginally posted by ChopperCharles Quote
You're wrong, I've tested with the chart centered, and at the edge so there is no field curvature to deal with.
I don't understand what you mean. If you test witht he chart center, then there *is* field curvature to deal with. And if you test with the chart not centered, the test is invalid, because you are not presenting an unambiguous focus target. Posting examples examples would help. But as I've said many times, it is quite difficult to perform a well-controlled, valid focus test.

QuoteQuote:
I've tested in natural and in tungsten light. Under the same test conditions, my 55-300 and my tokina 35-70mm, the two lenses I *always* get sharp shots with under all conditions, don't front focus one iota.
You don't see the FF in tungsten light on those lenses because DOF is too big. It's normal to only discover an issue with fast lenses for that reason. The mere fact that you are not seeing the FF that is guaranteed to always be there in tungsten light is proof of this.

Believe me, I've been down this road many, many, many times before, so I do have an idea what the common mistakes people make are. Posting test shots would help, but based on your descriptions, I sure don't see anything out of the ordinary.

QuoteQuote:
Tamron indicated they could fix it for $140, and their focusing problems are well documented on google, so I don't see how you can say the problem is the camera body.
Sure they will take your money and send the lens back to you. But I'm telling you, there are known physical and very obvious and practically unavoidable things that can be wrong with a camera to cause it to misfocus. It is very easy to explain and see exactly how this could happen. But there is practically nothing that could be misadjusted in a lens that would cause misfocus, and as I said, the two cases that have been proposed would be easy to test for. It's just that no one ever does

As for the problem being "well documented", well, yes, you are hardly alone in assuming a problem is with the lens. So there a plenty of other erroneous reports out there, too.

Mind you, I'm not saying I know for sure there is no problem with your lens. Just that nothing you've posted seems more consistent with that idea than with the more common explanation I've given.

07-14-2011, 09:05 PM   #20
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QuoteOriginally posted by Marc Sabatella Quote
But there is practically nothing that could be misadjusted in a lens that would cause misfocus, and as I said, the two cases that have been proposed would be easy to test for.
Marc, you are of course correct that often the camera and/or the user are the cause of the problem.

But there is also absolutely no denying that zooms can exhibit different FF/BF behaviour at their extreme ends. Even Canon acknowledges this and recommends to calibrate the long end. Here's a corresponding quote from Michael Tapes, the manufacturer of the LensAlign tool:
"Regarding zoom lenses - here we have to compromise because at present time most camera do not allow a separate setting for multiple zoom positions.

The exception being Olympus who does allow separate settings for the short and long end of the zoom range.
"
I admit that I'm not sure what exactly the cause is but as a matter of fact the phenomena exists (why would Olympus offer the feature if there weren't an issue?) and apparently can be addressed to some extent. Maybe a rotation of some lens elements can fix it, maybe a group is slightly out of alignment, etc. The AF subsystem doesn't "see" the same as the sensor and any variation in spherical aberration, for instance, will cause a varying degree of FF/BF.

You are certainly familiar with the "focus shift" phenomenon from fast lenses. This is also caused by the AF subsystem seeing something different (at f/5.6) than the sensor (e.g, at f/1.4). I reckon that a similar effect can be produced by a zoom if some moving group is a bit misaligned.

@Charles, you may want to check my AF adjustment hints for alternative ways of checking focus adjustments but judging from your descriptions, I anticipate they will only confirm the unfortunate behaviour of your zoom.

Last edited by Class A; 07-15-2011 at 05:16 AM.
07-15-2011, 10:26 AM   #21
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QuoteOriginally posted by Class A Quote
I admit that I'm not sure what exactly the cause is but as a matter of fact the phenomena exists
As I said, there have been theories proposed, and they can usually be tested for specifically (eg, by comparing results at different apertures to check for the "residual spherical aberration" theory, although I suspect that's more of a lens design issue, not something that can be adjusted).
07-16-2011, 04:34 AM   #22
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QuoteOriginally posted by Marc Sabatella Quote
As I said, there have been theories proposed, and they can usually be tested for specifically (eg, by comparing results at different apertures to check for the "residual spherical aberration" theory, although I suspect that's more of a lens design issue, not something that can be adjusted).
Well, if it were just a lens design issue then every copy of a certain model should exhibit the same behaviour. Judging from user reports they don't.

Also, even if it were a lens design issue, that would be reason enough to give a poster some credit for reporting their observations, rather than responding with "99% of reported problems turn out to be testing error" and then stating that apparently of the remaining 1%, in 99% the camera is to blame.
07-16-2011, 05:16 AM   #23
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I found out the reason at least 50% of my Tammy 28-75 shots where badly focused in the beginning... I used to use it pushing the speed/iso a lot, due to its 2.8.. This corresponded with me trying to often squeeze off a shot quickly, or stretching the friendship shutter speed wise... I also have AF setup to only function when the AF button is pressed, not on half-shutter press... Consequently, I found I a lot of the time was 'racing SR' and squeezing shots off without waiting for SR to have 'locked' (the hand appearing in the VF)...

At first, I was ready to swear at the tammy thinking it was a lens problem.. then I put two and two together and found I was using it places I just didn't *try* to use other lenses, and whenever I'd been patient or otherwise used it in more 'lens friendly' environments it had worked fine.

I've now almost kicked the habit of 'clicking too fast' and more often than not have that 1second pause required to ensure SR is activated when not tripod shooting.. and amazingly, my Tammy pictures have come good more often than not! I must have fixed my lens

Not saying that there isn't issues with the OP's particular Body/Lens... but my initial fears, further compounded when doing net searches and finding reports of bad tamrons with focus issues, ended up being mostly Operator Error.... and I now love my 28-75mm... If only it could help me actually compose pictures rather than just make crap ones sharp!

07-17-2011, 08:37 AM   #24
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QuoteOriginally posted by Class A Quote
Well, if it were just a lens design issue then every copy of a certain model should exhibit the same behaviour. Judging from user reports they don't.
I'm not convinced. We can't prove that most of the variation in reported results isn't just measurement error, combined, of course, with differences between cameras and how well their AF sensors are positioned and aligned.

The thing is, the optical theory would cause the lens to focus perfectly wide open and only show problems as you stopped down. Which of course would tend to be masked by greater DOF so it would be rare that anyone would notice. People post test results at different apertures all the time, and they virtually never show this behavior.

I'd have no doubt that the mechanical theory - gearing ratio not right - could indeed introduce some variation. This is the part I'd love to see people specifically test for, by posting results after a single focus operation but like I said, no one ever does.

Those are the two theories I've seen proposed. Until someone demonstrates another actual physical cause that could be in play, I think it only natural to be skeptical. You've heard of "voodoo ecomomics"? To me, this is "voodoo focusing" :-)

QuoteQuote:
Also, even if it were a lens design issue, that would be reason enough to give a poster some credit for reporting their observations, rather than responding with "99% of reported problems turn out to be testing error" and then stating that apparently of the remaining 1%, in 99% the camera is to blame.
Point taken. I think my initial posts were meant to do that, by observing that the described results sound normal to me and explaining why.
07-17-2011, 09:15 PM   #25
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Well, after adjusting the focus -150 I'm getting pretty much consistently correct focus lock. it still front-focuses a lot more at 75mm than it does at 28mm, which makes the -150 adjustment a big compromise, but I was still able to pull off these two shots the other day!

approx 5" from lens (Wing was chosen target, blur of body was intended):


approx 12" from lens:


Charles.
07-21-2011, 10:25 AM   #26
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I'll cast my vote for a fault in the lens. I recently bought a new Tammy 28-75, it had the same problem (in full daylight, real-life photography) and to my small surprise it was quickly exchanged for a new copy, they didn't even try to fix it. The new lens is much better (although it's still impossible to get it sharp over all distances, apertures and focal lengths at the same time).
And yes the 17-50 exhibits the same problem - I'm on my third one of those.
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