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11-26-2008, 03:48 PM   #16
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QuoteOriginally posted by jsherman999 Quote
Check out the terrible de-centering in these shots:

*snip*

Terrible, huh? (Moral of story: I have no idea, nor do I care, how my lens would
capture newsprint.)
Ah. It was good of Kirivon to let you use his lens to use in those shots. How long did you have it? How did you get it back to him?

Kirivon also posts comparison tests with a cheaper, supposedly inferior lens, the DA 50-200, which, in my mind, is sharper across the frame and lacking the chromatic aberration of the DA*.

In other words - he makes a pretty good case. And for ~$750 US, I think he's got the write wonder.

11-26-2008, 03:52 PM   #17
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QuoteOriginally posted by flyer Quote
The DA* 50-135 is not a macro lens, and as such, it might have a very hard time rendering a flat subject at close range 100% sharp corner to corner. I really would try to check if you slightly alter focus forth and back to see if that part that is not perfectly sharp can be brought to sharpness. That would mean that focus at close range is not perfectly flat, not necessarily a de-centered lens. Macro lens and general purpose lenses are not designed with the same goal, which might explain your results.
Although the 50-135 is not a macro lens (1.0m minimum focus distance), neither are the 50-200(1.1m) or the M135 f3.5(1.5m).

I do not expect a constant f2.8 zoom to be sharp corner to corner, nor am I grieving its edge performance. What I was concerned with was the apparent disparity between the left and right sides, which was something that no amount of altering the focus would change.

Klaus mentions that the lens doesn't perform as well at distances under 1.5m and this was taken into consideration. The newspaper shots were taken between 1.3m to 2m away from the wall.
11-26-2008, 04:13 PM   #18
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QuoteOriginally posted by philbaum Quote

there is some dramatic fall off of border resolution reported in that test, e.g. at 135 F2.8, the center resolution is 1915 and the border resolution is 1494, thats almost a 25% of resolution lost between the center and the border. But then again, there isn't one lens that doesn't have falloff of resolution between the center and border.

there is no perfection in this world, there is a manufacturing tolerance on everything. For a zoom sealed fast lens, i am amazed they can produce something this good.
As am I. If I came across as if I were bashing the lens in my opening post, that was not my intention. This lens is the reason I savor any news of the K30D, and this lens is the reason I wouldn't consider anything other than a Pentax. For me, there are two single zooms that justify an entire camera system. One is the ridiculous nikkor 14-24 f2.8. And the other? You guessed it.

Sure, the DA*50-135 is available on other mounts. But, without SR, weather sealing, and SDM it makes other camera bodies much less appealing. And, I really love that Pentax has embraced the APS-C format to give us something with such a useful focal length at half the weight, size, and cost of the competition. The limiteds are just icing on the cake.

I know this lens performs the worst at full tele. Looking at the lens makes this fact obvious: the elements all open to the maximum to swallow up as much light as possible. It was for this reason that I showed most shots at around ~100mm, and left out comparisons with the M135 F3.5, which would frankly be unfair and irrelevant.

Anyhow, I've been hard at work almost all day today doing test shots. Unfortunately it has been raining so I couldn't do the real-world comparisons that I've been wanting to do, but I did do some non-newspaper tests that put the newspaper results into perspective. My initial post was perhaps a tad premature but, the results of that test were pretty devastating and I needed some encouragement to keep going. I will edit the opening post once I have everything together.
11-26-2008, 04:19 PM   #19
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QuoteOriginally posted by Kirivon Quote
Although the 50-135 is not a macro lens (1.0m minimum focus distance), neither are the 50-200(1.1m) or the M135 f3.5(1.5m).

I do not expect a constant f2.8 zoom to be sharp corner to corner, nor am I grieving its edge performance. What I was concerned with was the apparent disparity between the left and right sides, which was something that no amount of altering the focus would change.

Klaus mentions that the lens doesn't perform as well at distances under 1.5m and this was taken into consideration. The newspaper shots were taken between 1.3m to 2m away from the wall.
I believe that 20x the focal length is the conventional distance for doing the newspaper test. At that distance any well-designed lens should not have significant field curvature.

Looking at your example, there does appear the lens does appear to have a centering defect. The clue is not the blur. Rather it is the fact that the blur is much more evident on the left side. Three things to consider:
  • Are you sure that the newsprint was flat (glue might be appropriate here)
  • Are you sure that the focal plane was parallel to the subject
  • If you tear down and re-do the setup are the results reproducible

The second point cannot be overemphasized. It is rather difficult to precisely align camera and target. Again, a little more distance helps here.

One test protocol that I had read suggested doing a focus series at a set distance. At the extremes of the series the full subject should be out of focus. Focus should improve to the point of being in focus. Field curvature will show as the margins showing different focus than the center. With poor edge performance, the margins are never sharp. Centering defect is present if focus is asymmetrical. A poorly ground element may also mimic centering defect.

Steve

11-26-2008, 04:33 PM   #20
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QuoteOriginally posted by stevebrot Quote
[*]Are you sure that the newsprint was flat (glue might be appropriate here)[*]Are you sure that the focal plane was parallel to the subject[*]If you tear down and re-do the setup are the results reproducible [/LIST]
The second point cannot be overemphasized. It is rather difficult to precisely align camera and target. Again, a little more distance helps here.
Steve
I agree that it is very difficult to place everything in proper alignment. Today's testing was done with a cereal box at about 10m, using the grout lines on the tile flooring to get everything into correct alignment.

As for the newspaper tests, I had done an initial hand held test on the south side of my bedroom with no real consideration to flatness or perpendicularity. I then repeated the test several times on the north wall, this time with careful attention paid to the paper's flatness and tripod's alignment.

In all tests the left 15% of the frame lags behind the right 15% to varying degrees. As it turns out, the DA50-200 is also decentered, as it is much sharper on its left side than its right. It was kind of a perfect storm that made the DA50-200 seem a lot better than it actually is--an untruth my new test rectifies.
11-26-2008, 05:46 PM   #21
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This is why I don't shoot newspapers, or other "test" shots. I either like the results of the lens in real world conditions, or I don't.

I just don't like to go looking for a reason to be dissatisfied with my gear.
11-26-2008, 06:48 PM   #22
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QuoteOriginally posted by rfortson Quote
This is why I don't shoot newspapers, or other "test" shots. I either like the results of the lens in real world conditions, or I don't.

I just don't like to go looking for a reason to be dissatisfied with my gear.
That's how I see things, too. The real world pictures are what I'm looking at, not newspaper pictures.
11-26-2008, 06:53 PM   #23
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QuoteOriginally posted by flyer Quote
That's how I see things, too. The real world pictures are what I'm looking at, not newspaper pictures.
exactly my point as well.

11-26-2008, 07:26 PM   #24
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Let me try to bring some sanity to this Kirivon/newspaper bashing fest:

A DA* lens, at around 2m, even if it's at the end of its zoom range, should be able to show consistent across-the-frame sharpness at f/8. Period. You don't pay $750 for a lens that's decentered (Kirivon mentioned that no amount of trying could bring that left side into focus, which rules out field curvature).

Please stop saying you don't take pictures of newspapers. Neither do I in my everyday photography, but what if Kirivon uses his lens to take a shot of his family lined up in front of their house? Whoever is standing on the left side of the frame is going to be blurry. Hopefully, it'll be the weird aunt that nobody likes, but what if it's Kirivon's dear grandmother?

The point is, if the lens is decentered, it can affect the quality of pictures taken in the real world. It might not affect every picture, but does anybody want a lens that you have to keep reminding yourself not to use in certain situations because it's faulty? I certainly don't, and least of all if I paid $750 for it. It's one thing to understand what the inherent shortcomings of a lens are when they are due to the nature of its design (not sharpest wide open, not sharpest at closest focusing distance, etc.) and avoid them. It's quite another thing to put up with a construction fault or defect.

Kirivon, if this were my lens, I wouldn't return it, I would send it to Pentax to have them fix it under warranty. Call them up and ask them what you need to do (they'll tell you to include photos showing the problem and a full description).

Shooting newspapers, brick walls or what have you is an integral part of testing a new lens. If you guys don't want to do it and ignore a possibly faulty lens, that's up to you, but please don't tell someone with a faulty lens to shut up and go shoot.

Would you tell someone in a car forum who reports his new car doesn't brake properly to shut up and go drive?

'Nuff said.
11-26-2008, 08:27 PM   #25
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If you want to try the newspaper test, then take the picture with the camera fixed on some boxes or table, then take another one with the camera upside down. This way you are fixed in the same position and can see if it is consistent. The images should be less sharp on the right hand side when upside down to be consistent with your assumption.
11-26-2008, 08:29 PM   #26
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Nor is this the 1st instance I'm reading of of this.... https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/41432-testing-...-50-135mm.html
First the 16-50 "good copy hit-or-miss woes", now this... :ugh: I had thought I was going to get this lens a few weeks ago. Do we need to ask for a sticky tracking the 50-135mm serial numbers like the 16-50mm too?
11-27-2008, 04:34 AM   #27
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QuoteOriginally posted by jsherman999 Quote
Check out the terrible de-centering in these shots:

You're right. Their asses are all different.
11-27-2008, 04:35 AM   #28
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I always try to get my photographs in newspapers, rather that the other way round.
11-27-2008, 03:09 PM   #29
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New test added to the op.
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