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Old Jan 28, 2010, 12:43 PM   #1
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Default Back with my De-centering issue

Ok, I shot this grid only about 5-6 feet from the paper full 300mm, one at f5.8 the at f20. I'm sure at that close if i'm not dead square I'll have depth of field issures. the f 20 looks good all the way around. the f5.8 looks soft along the bottom , and slightly more on the left. Could this be that the camera was just not square enough at that close distance?
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Old Jan 28, 2010, 1:00 PM   #2
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I can't say with any authority Tony but, it may be an issue common with a lot of zooms.
They are optimized at a certain f:stop setting and zoom range, everything else is trade off.

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Old Jan 28, 2010, 1:28 PM   #3
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Here are two more shot closer to infinity. I was spot focused on the wall.
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Old Jan 28, 2010, 5:55 PM   #4
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Hi Tony,

I'm not big on trying to test lenses like this, preferring to just go out and shoot a lens in the field. My biggest problem is not having the patience to set up tests so they are actually valid.

You've correctly questioned the close distance test. Anything less than exactly square will
effect the results, especially when the differences are so small. I'd suggest that you stick a small mirror on the wall, mount the camera on a tripod, then aim the camera so your view in the VF is looking straight into the lens of the camera's reflection.

In the second test, the intervening branch might cause some confusion for the AF system in the first shot (assuming center point focusing), but in the second shot, the branch would not be a factor. I realize that What the AF focuses on is not the issue, but when determining what is "sharp" in the frame, you want minimal focusing distance differences between the shots to eliminate possible DOF issues (by this, I mean that in one shot, you may have the wall in the center of the focus plane, but in another, the wall might be at the edge of acceptable focus within the lens' DOF, which would show the effects of camera misalignment more critically). The fact that the two shots aren't framed exactly alike shows that at least one was shot when the camera wasn't square to the subject. Also, in the first, the leaves on the branch are focused, but they aren't in the second.

This is the kind of thing that makes my lack of patience a liability to testing like this, so I've pretty much given up on doing it. Usually by the time I finish testing, I've already come up with enough flaws in my testing that it ends up to be a wast of time. . .

All that being said, do you really see a real practical problem with this lens in your normal use? I don't think I would from what I see here. . . I shoot a lot of tele, and for the most part, I center the subject and need critical focus there -- and the rest of the frame, except for possible background considerations -- doesn't much matter to me. In other words, in a tele, I look for center sharpness/resolution, and pretty much disregard edge and corner performance, though the nature of the bokeh might be a fine point to consider in my appraisal of a lens' performance. For me, the purpose of a long tele is to isolate the subject within the frame -- to pretty much focus attention on one major element in the image.

For normal to wide shots, which I'm admittedly not very good at, I like to see better edge/corner resolution, as I'd be looking to capture a vista, and the whole frame tends to be the subject, rather than one object in particular.

I'd suggest you just go out and shoot the lens as you had originally. Take a lot of images (at apertures that you would normally use -- this is important -- don't just shoot wide open, unless that's how you originally intended to use the lens), and see if they meet your expectations. If you see that they are consistently unacceptably soft at the bottom and left side, then you likely have a decentering issue, and should have the lens repaired or replaced. If the "problem" doesn't show up in normal shooting, then I'd forget about it and enjoy your lens.

A lot of the problems discussed in photo fora seem to get a lot of people to really examine their gear. I think that this is good in the sense that some issues are discovered that need to be addressed by the mfg. This is also not so good, because it sometimes causes faults that are within normal mfg tolerances to be seen as major flaws that need to be rectified when they really don't qualify -- and it sometimes gets people to see faults where they really don't exist.


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Old Jan 28, 2010, 8:47 PM   #5
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It seems that there is some vignetting in the second shot that would concern me me. If you are certain that the camera was perpendicular to the grid, then the corner softness would also.

I understand the K7 has some lens corrections in it. Do you have this feature enabled for the lens? If so, do you have the latest firmware update? These could make some difference in your results.

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Old Jan 28, 2010, 9:51 PM   #6
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This looks to me like nothing more than the effect of shallower depth of field at f5.6 with a camera that is not completely square with the paper. You really can't do a proper rigorous test without an optical bench. You could take exposures with the camera adjusted slightly up, down, right, and left and see if you don't find the perfect shot. You chould also try other focal lengths at which you have greater depth of focus. Or forget about it and don't do anything but use the lens under actual shooting conditions - Scott's advice is good. You may be looking for an issue where no significant one exists.
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Last edited by penolta; Jan 28, 2010 at 9:56 PM.
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Old Jan 28, 2010, 10:35 PM   #7
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I admit I'm not a pixel-peeper, so I have little experience evaluating these sorts of tests. Either a picture works for me or it doesn't. By my standards, all of your images look fine. Sure, maybe one aperture is a hair sharper than another, but as Scott pointed out, in day-to-day photography, those very small differences don't matter.

Of course, I'm not a fine art photographer, intending to make very large prints. If I were, I probably would concern myself with those issues. So it depends on how you plan to use your images. For whatever it's worth, I'd say don't split hairs over these sharpness issues -- just go out and shoot some pictures. (So says the guy who hasn't picked up his camera in three weeks!)
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Old Jan 28, 2010, 10:48 PM   #8
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I've looked at these pictures a couple of times. The second one in the first post is the only one that seems to have a difference between left and right, with the left being slightly blurrier. The wall looks to be about the same on both sides. I find it difficult to see such things with resized pictures, it's much easier to see it when you look at the full sized image. If you go back and forth about it, then try doing a 100% crop from each side and putting them side-by-side, leaving the center out of it. These pictures don't convince me that your lens is defective.

Also check for focus issues - if your camera is back-focusing with the lens and you aren't quite straight to the wall, then it might appear as decentered when it's not.

If you really think the lens is defective, send it in to Pentax for warranty repair. They'll look at it and send it back if there's nothing wrong with it. If it really is defective, they'll either repair it or replace it.
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Old Jan 28, 2010, 11:02 PM   #9
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Thanks everyone for your answers. I will shoot a bunch of shots at infinity, and look at them closely. After all this is not a high end lens, and this may be with normal operating limits. I'm actually getting tired of messing with it.
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