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11-11-2009, 12:08 PM   #46
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QuoteOriginally posted by WheresWaldo Quote
Now I understand where we are different. I did not mean to imply that the thin line was the issue, and I don't think I used those exact words. What I said was the target is too small. The target becomes even smaller the more oblique your angle is and also becomes smaller the further away you are. The smaller the target area the less likely that you will know where you are focused.
OK, so it sounds like you *are* talking about the fact that the box around the center line could potentially end up within range of the AF sensor if you are not careful? If so, then yes, we are in agreement on that count. I just don't see it as a reason to throw out the chart - I see it as a reason to be careful not to get the box within range. It does, admittedly, limit you to only testing at close focusing distances. So I can certainly see the value of having other methods available too. But I still like this one for its simplicity.

Anyhow, I'm glad we seem to have sorted out the misunderstanding - I'm sure it was confusing and frustrating you as much as it was me!

11-11-2009, 12:23 PM   #47
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QuoteOriginally posted by rkt Quote
(1) are the tests (and the results above) conclusive either ways? What is your conclusion if you got these results?
The color corrected shots from a closer distance at 50mm are certainly illuminating, as they suggest that maybe the camera really *was* focusing on the "Focus test chart V2.00" line previously. One thing I can't remember if you said or not - you *are* choosing the center focus point here, rather than letting the camera choose where to focus, aren't you?

Anyhow, those last two shots appear to show if there is a problem, it's *much* less severe than the initial tests had led us to believe might be the case. I would concentrate on repeating those tests - bumping up exposure by slowing the shutter (you are, after all, using a tripod, right?) - at a variety of different focal lengths. Always beign careful to make sure the text isn't in range of that central AF point.

My view is that that if the target appears anywhere within the zone of acceptable focus, there's no point in worrying further. It's kind of hard to tell at these reduced sizes, but it appears that *may* be the case in those last shots. But since the results are so obviously different from the previous ones, I'd still be assuming my own error was leading to the inconsistency, and work on eliminating that error. And I'd start with assuming I really *wasn't* close enough - or was shooting too extreme an angle - in the first set.

I suppose I misled somewhat when I suggested it only takes a minute to get a completely reliable test done. That's true once you've made every mistake in the book and learned from them all how to avoid making those same mistakes again. After a couple of months of obsessive focus testing last year with a variety of cameras and lenses, I became quite good at it. It made me a better photographer, too, as the things I would do wrong that led to inconsistent resuls on test charts *also* led to inconsistent results in real life.

So for me, now, running a good reliable focus test takes only a minute, and I have confidence I'll nail it my first attempt. But you might still have some experimentation to go. Looks like you're getting closer and closer, though, gradually sorting out the things that *don't* work.

I have tried to remove this effect by throwing the lens off-focus before each shot... by focusing on something different and then refocusing on the target line...
Yeah, that's good technique in general. But if I got to the point of suspecting a (probably rare) lens problem as opposed to (probably much more common) camera problem, I'd try doing hat I suggested to see what happened. If it did get better with each successive try, finally ending up good and staying good, I'd be more inclined to blame the lens than the camera. Otherwise, it would be very hard to convince me the lens could possibly be at fault, and I'd keep testing with other lenses to see if perhaps testing error wasn't masking a problem there too. It might take a while before I could finally accept the conclusion that the lens was at fault.
11-12-2009, 02:53 AM   #48
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QuoteOriginally posted by Marc Sabatella Quote
you *are* choosing the center focus point here, rather than letting the camera choose where to focus, aren't you?
Yup, all shots are taken with the center focus point selected... and the AF test chart shots were taken using a tripod as well...

QuoteOriginally posted by causey:
Mathias (Kameraten) kindly agreed to my posting his reply
Thank you for sharing his inputs... I will try and contact him for some timelines to see when he had this experience... to check if my lens was after the product development he mentions.

QuoteOriginally posted by Ash:
Do you get similar focusing troubles in real life images, both close and far focusing
I got a chance to finally take some outdoors snaps with some morning sun. By the time I get home it is usually very dark... I have put up 10 snaps on the following flickr link... please don't evaluate them on composition etc as I just got up on time to have 15mins for a few quick shots to post before getting to work...

If you feel some other test subjects make more sense, then do let me know...

Tamron 17-50 test shots - a set on Flickr

All the snaps are at 2.8, in Av mode... handheld... morning sun... ISO 100...

I try and embed here as well...

Focussed on the wall, 17mm

Focussed on the wall, 50mm

Focussed on the BMW tower in the middle...extreme right band due to intruding window

Focussed on the first (from left) of the four attic windows on the red tiled roof...

Focussed on the white chimney (in the middle of the pic), on the front brown tiled roof...

Focussed on the middle of the clump of leaves (and the bud?) in front...

Focussed on the ribs (underside) of the leaf on top - left (vs. other leaves)

"to keep, or not to keep; that is the question..."
11-15-2009, 10:27 AM   #49
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Still looks sharp.. Does the Tamron 28-75mm have front focus issue with the K200D also?

11-16-2009, 10:55 AM   #50
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is this front focus problem with the tamron 17-50 happening on the sigma 18-50 as well with a k200? is front or back focus a non issue with pentax lenses?
11-16-2009, 11:21 AM   #51
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Just to chime in a bit, I have the Tamron 17-50 lens with my K10d, and it serves as my primary lens for probably 80% of my shots. I have never had an apparent FF issue, but I have had the issue that Marc has described in this thread with the size of the AF area and the lens choosing the wrong item to focus on. I've actually always assumed that was the problem as that is how I am with lenses and working with much worse lenses and cameras in the past.

I do find I have more trouble when I am working with a shallower depth of field, but I still often attribute it to AF confusion (flowers, etc). Usually when I get home I find that 90% of my shots have sharply focused on what I've wanted.

Overall, I've found my 17-50 lens to be the best zoom lens I've ever owned and it easily shames other Tamron lenses I've owned. Doesn't mean there aren't better out there (I'm an amateur and haven't tried many), but for its value I've loved it.

Of course, it could be that I have a rare good lens, but I still find that with most lenses my K10d does have a big focus area that often picks up focus on items I don't always expect it to pick up. That really becomes obvious to me though when working on a bigger scale... I just saw it the other day shooting at my uncle's farm where the camera picked up focus on the wrong fence post as I was shooting a shot along the fence for perspective. It was obvious that the wrong fence post was picked up as opposed to front or back focusing.
11-16-2009, 12:26 PM   #52
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Independent of the focus, the distortion on that brick wall shot is pretty ugly... I guess that's par for the course with this particular lens, though.
11-16-2009, 03:04 PM   #53
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Yup, I agree about the distortion...

The lens is still with me... I plan to get some more natural light shots before judging it... just that this time of the year, sunlight is a rare commodity and it's been mostly overcast and rainy when not outright dark...

Any thoughts on the focus though?

11-16-2009, 10:01 PM   #54
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Not really. The tests posted thus far seem inconclusive to me. But it's way too hard to diagnose problems like this long distance - I can't force you to run exactly the tests I'd want you to run, exactly the way I'd want you to run them.

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