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03-17-2016, 10:34 PM   #1

Join Date: Oct 2007
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Do I have a lens with decentring issues?

I have a Pentax DA* 16-50mm lens that I think has a de-centred element.

I've micro-adjusted the focus to eliminate any back and front focussing issues but I see softness away from the centre where it shouldn't be even when the lens is stopped down. For example, if I take a photo of a group of people the people in the centre will be sharp but the people at the ends of the group will be out of focus even though they are equidistant from the camera.

In the sample image, the building is relatively in focus in the middle around the White Lion sign writing but at either end the image becomes soft. Itís like Iíve applied a blur filter and masked a big circle in the middle.

My technique may not be the best but the softness is present when using phase detect auto focus, contrast auto focus, and manual focussing.

Iíve had the lens for too long to send it back for a replacement so I'd like to have it looked and hopefully repaired by a professional repairer that will do it right but I want to make sure I am telling them the right thing first.

03-17-2016, 10:58 PM   #2
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Some test shots of a flattish subject parallel to the sensor plane would be helpful. Brick walls are of course the traditional choice.
Use a tripod, CDAF and 2 second delay on the shutter. Use a variety of apertures, focal lengths and focal distances. Take note of where the problem is worst, and if there is a sweet spot where it is fine.

If you can't find a practical work around, then it's certainly time for a service, or replacement.
03-18-2016, 03:54 AM   #3
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QuoteOriginally posted by Mike L Quote
, if I take a photo of a group of people the people in the centre will be sharp but the people at the ends of the group will be out of focus even though they are equidistant from the camera.
Ah, but they are not equidistant from the camera, unless they are standing in a half-circle.
Don't confuse lens distortion for decentering. And don't confuse edges being slightly softer than centre for a problem (this is something pretty much every lens does). Also, most lenses have a curved field of focus, not flat (so if you focus on a flat wall that is parallel to the sensor, the DoF will not run along it), and it might even have its own shape. I think pretty much only macro primes have a very flat field of focus.
So sample photos would help, of something like a newspaper parallel to the sensor's horizontal axis, but tilted 45 degrees up. Use ISO 100, manual focus, place the middle of the paper near the lens' minimum focus distance, use Av and wide open aperture.
It is possible you have a decentering issue, but we have to eliminate other possibilities
03-18-2016, 04:17 AM   #4
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what i noticed very recently when looking into the tube of my 16-50, the actual outside-tube of the lens does not rotate, but the inside focusing mechanism does.
making it very hard to detect decentering issues, as -depending on where your point of focus is - soft edges may at a time be at the far vertical axes or at the far horizontal axes or anywhere in between.

im not sure about my interpretation, so i'd be happy if somebody else could confirm this

03-18-2016, 06:20 AM   #5
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Easiest (and still reasonably reliable) way to find out is to focus to infinity and point the lens at a tall object far away, like a tree, tower etc., then taking four consecutive images with the object in all of the four corners of the frame. Then zoom in and compare, if the object is considerably less sharp in one image, you got decentering.
03-18-2016, 12:32 PM   #6
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what aperture did you take the picture at? you probably need to go with bigger number to account for the distortion like others said. this will give you a wider dof to work with.

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