[AlexL]

The results are in, and the results are...my K100D body might actually be back focusing slightly!

I tested the Tamron using a PDF chart I got from the forums. I forgot the link, but it had the chart with millimeter graduations and required that I take photos focused on the center black bar at a 45 degree angle.

Anyway, it seems to be back-focusing, but I decided to do the same test with my Pentax 16-40 f4 just in case. I got the same results with both lenses, the sharpest focus seemed to be a little bit behind where it was suppose to be. I figure that the odds that both lenses are at fault is probably low, so my K100D body is more likely at fault.

I'm a little bummed, but not terribly since I don't really rely on such narrow depth of field which is why I never noticed this before. I'm gonna try to ring up the warranty with Pentax in a few weeks since I need my camera for the next several weekends. I won't be needing such narrow depth of field in my next photo ops, so the slight back focusing won't affect me.

I'll try to post some photos of the tests later, and test other aspects of the Tamron over the weekend. So far, the Tamron seems to be fine

[hudsong]

Man, I'm getting absolutely sick of how many issues I'm seeing with Pentax systems misfocussing. Body, lens, user error, whatever it is, I have simply never seen this many complaints about focusing issues with any other DSLR system (I've owned a Canon, have friends who use Canons and have friends who have Nikons). I'm starting to wonder if switching back to a K10D was a good idea, even though the most important thing I do is a band shoot or wedding here and there.

[Torphoto]

Other systems back focus to, it's not just pentax. At least with the K20D you can adjust it now.

[k100d]

The results are in, and the results are...my K100D body might actually be back focusing slightly!

if it's not a K100D Super, you can adjust the AF. my K100D backfocuses too,

[FotoPete]

Aren't we getting a little ahead of ourselves. Is such an issue prevalent in outdoor tests as well? I mean depending on certain spatial variables like camera position, more than one of those line may fall into the centre AF area and so its it really "FF" or "BF"?

[AlexL]

I've heard of focusing issues in other camera systems too, even when I was on Nikon. Back then I never bothered doing focusing testing like this, so I could have had the problem and not notice it.

Anyway, thanks k100d for the clue, I found this through Google:

self focus adjust for dl,ds,ds2,k100d.: Pentax SLR Talk Forum: Digital Photography Review

I played with the debug mode a little, and I seemed to have gotten an improvement. I'll play with it more later, I gotta finish my taxes right now. I also gotta test to make sure that any AF adjustments I make in testing conditions don't worsen real-world photo taking. Honestly, I'm actually impressed, I never expected to be able to correct little issues like this on my own!

[247nino]

Man, I'm getting absolutely sick of how many issues I'm seeing with Pentax systems misfocussing. Body, lens, user error, whatever it is, I have simply never seen this many complaints about focusing issues with any other DSLR system (I've owned a Canon, have friends who use Canons and have friends who have Nikons). I'm starting to wonder if switching back to a K10D was a good idea, even though the most important thing I do is a band shoot or wedding here and there.

I think Canon wins in the contest to have focus problems,

ahem 30d, 40d, and mk3s...

Ive owned a canon and my friends shoot Cannot, so I can say this

[roscot]

How much is it back focusing? If focus is with 2mm in front or back it is within spec.
Also, were you using tungsten lighting? That will also effect the focusing spot. Try the test using daylight.

All makers AF systems can have BF/FF issues. To some extent, they rely on DOF to mask the BF/FF. Of course if you are shooting with a fast lens wide open, you don't have any DOF. That is why you are beginning to see focus adjustments in the more advanced cameras.

[hudsong]

Maybe Pentax users are just overly attentive to this issue then. I had an issue with backfocusing, so I downgraded firmware and fixed it (K10D), something other systems don't allow the user to do so easily, I think...

[k100d]

I played with the debug mode a little, and I seemed to have gotten an improvement. I'll play with it more later, I gotta finish my taxes right now. I also gotta test to make sure that any AF adjustments I make in testing conditions don't worsen real-world photo taking. Honestly, I'm actually impressed, I never expected to be able to correct little issues like this on my own!

try it out on a focus chart with the lens wide open at the longest focal length to get the narrowest DOF (assuming this backfocus is constant). for my backfocus, i adjusted by +100

[jake123]

I've heard of focusing issues in other camera systems too, even when I was on Nikon. Back then I never bothered doing focusing testing like this, so I could have had the problem and not notice it.

Anyway, thanks k100d for the clue, I found this through Google:

self focus adjust for dl,ds,ds2,k100d.: Pentax SLR Talk Forum: Digital Photography Review

I played with the debug mode a little, and I seemed to have gotten an improvement. I'll play with it more later, I gotta finish my taxes right now. I also gotta test to make sure that any AF adjustments I make in testing conditions don't worsen real-world photo taking. Honestly, I'm actually impressed, I never expected to be able to correct little issues like this on my own!

Please tell us how it goes! There's hope for good copies!

Maybe Pentax users are just overly attentive to this issue then. I had an issue with backfocusing, so I downgraded firmware and fixed it (K10D), something other systems don't allow the user to do so easily, I think...

Go to Fred Miranda and take a look to this lens to see how it also had some issues in those systems. I think that people is attentive everywhere the main difference is that with Canon and Nikon they may produce different batches of lenses cos they sell more and they might adjust some problems in subsequent editions. But this is pure wondering.

[AlexL]

I was able to do more extensive tests this weekend, and in better lighting too since I was doing it during the day. I did the same tests with my Pentax 16-45mm f/4, which is I think is a fantastic lens even though it's a little on the slow side. I'll mention the Pentax lens a lot since I think it's a fair comparison.

The good news:

With the better lighting, I got better results with focusing, so I may have overreacted when I started talking about back-focusing. I'm actually not sure about this now, but depending on how I test, I either get a little bit of BF or none at all. The Tamron is behaving like to my Pentax lens, so I'm declaring this as a non-issue.

I did some informal sharpness testing looking out my balcony, and the lens seems pretty sharp. Even wide-open and approaching the corners, details look crisp. Sharpness seems pretty on par with the Pentax lens, which is quite sharp. I didn't do any thing like a newspaper-on-wall test.

I actually took a few hundred photos with it today at a dance studio, and the lens performed quite well. A friend asked me to take photos during the practice routines for their up and coming show, and I used the Tamron most of the time. With the somewhat low indoor lighting I was mostly using f2.8 and f4, so the underexposure issue doesn't rear it's ugly head at those aperture.

Now the bad news:

The lens starts to underexpose dramatically at anything other than wide open. The exposure taken at 2.8 looks "right" and on par with the exposures on the Pentax. When I start to stop down, the exposure gets progressively darker until about f22, which I'm estimating (through post-processing) is about 1.66 stops darker than at f2.8! That's nearly two-stops darker! I did the same test with the Pentax, and the exposure is pretty even until f16 and f22, and then it's gets a little dark, but by less than half a stop. This is a pretty serious flaw, which I assume is something that would be covered under warranty.

A few little things:

The focus throw is really short, so it's harder to manually focus. Maybe 60 degrees of turning room? Compare that to about 90 degrees of turning on the Pentax. There's no quick-shift in the Tamron either. I'm actually surprised how spoiled I am with quick-shift.

The Tamron build feels quite solid. The zoom movement is very tight but smooth.

The Tamron hood doesn't include a removable tab at the bottom. The Pentax hood as a removable tab, which I love being able to remove it to conveniently stick my finger through to turn the polarizer.

Vignetting seems a little more noticeable in the Tamron, but this is rather minor to me since it's easy to correct in post-processing.

So what am I going to do?

The Tamron was excellent in all the ways I tested it except for the aperture issue. It's a very serious blow, I often take pictures at f8 and slower. If for some reason I only took wide open all the time, then lens is excellent.

I was thinking that I could use the warranty to get Tamron to fix the underexposure issue, but that's gonna be a little bit of a hassle. I'm going to return it for a refund. I'm sure that there are good copies of the lens out there, but I'm very happy with my Pentax 16-45mm—even if it is a stop slower. I'm willing to wait it out, I don't really feel like dealing with any back-and-forth that might happen with bad copies.

I got too impatient waiting for the Tamron lens, but now I feel like I could wait a while before trying again. I'll definitely look into the Tamron 17-50mm some months from now to see if they've fixed it. I'll also give the Sigma 18-50mm 2.8 lens a serious look too, although I would have to upgrade my filter setup if I choose the Sigma (72mm vs the Pentax/Tamron's 67mm).

[bentax]

So who has actually got a good copy of this lens?
Ive been hanging out for it since it was announced... it was going to be my main zoom... and then all these focusing issues came right out with it.
Almost might aswell gamble on the pentax 16-50 2.8 if theyve both got qc issues?

[roentarre]

I have simply never seen this many complaints about focusing issues with any other DSLR system (I've owned a Canon, have friends who use Canons and have friends who have Nikons).

Most of my canon user friends worship canon as a religeon - not quite objective for sure. All the canon camera bodies I had owned had issues with backfocusing. Sometimes the camera did not even autofocus on anything at all under dark lighting. It was only the speed of autofocusing being relatively fast with many of the fast primes like 135/2 100/2 200/1.8 300/2.8 100/2.8macro; the difference is that people are allowed to complain about any pentax lenses in this friendly forum while canon users are to be banned or verbally bashed to death if canon lenses were criticised in various major gear fora.

My Da* 16-50 is not a perfect zoom, both backfocusing in low light and purple fringe wide open. However, it is a weakness to know and be more aware during shooting. I certainly rate the zoom above other lenses I had tried for its versatility in harsh weather condition without any concern for potential harm to my camera gear.