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Old Apr 3, 2011, 9:44 AM   #1
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Default Is there Anything Wrong with my Lens?

Hi All,

Hope you can help me out. This is the first time I encounter this kind of problem. I'm shooting with the DA 60-250mm and the Pentax Af 1.7x adapter

W/o Adapter, ISO 800, 1/800, f4


With adapter, ISO 800, 1/400, f6.7


I notice that the clothing outlines of some people in the shots has some kind of 'ghostly effect' when viewed at 100%. Is there anything wrong with the lens, or has it something to do with the adapter?
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Old Apr 3, 2011, 10:18 AM   #2
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Morning, Well its called purple fringing or chromatic aberration and you can remove it in post processing. Its caused by a lens' failure to focus all of the color spectrum equally on the sensor's surface.
It looks like in your case the addition of the adapter caused it. Pentax's 12-24 has this condition, so its not necessarily a bad lens design.

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Old Apr 3, 2011, 1:05 PM   #3
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Hi calvinlo,

Actually, most lenses will show a bit of purple fringing and or CA, especially at the edges and corners, and usually it's more evident at larger apertures and with slightly out of focus areas. I believe you'll see a bit of it at the edge of the girl's ponytail on the first image, and there is a trace at the top of her head and at her right hip. Any TC (the AFA wasn't designed as a TC technically) will tend to magnify the aberration because that's what they do. Zooms have a higher likelihood of showing CA/PF because there are necessary compromises to lens element design.

Low Dispersion (Achromatic) lens elements control this to some degree, but It's difficult to totally eliminate it at very high contrast borders. Of my premium lenses, the Sigma EX 180 APO DG is the closest to totally controlling color aberrations like this, and it still shows some at the highest contrast borders. My most expensive lenses, the FA* 300 f2.8 and Sigma EX 300 f2.8 both show CA/PF under some circumstances.

Some people obsess about this, but I choose to just find it mildly inconvenient. If I'm presented with this kind of contrast, I'll try to stop down the lens to help prevent it, and if this is not enough, I'll deal with it in PP.

Scott
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Old Apr 3, 2011, 7:11 PM   #4
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Thanks so much for the replies. I appreciate your help.

Ironically, I've heard about CA or PF in the past, but this is the first time I've actually encountering it. Perhaps the lighting was too hash when I did the tests.
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Old Apr 3, 2011, 9:02 PM   #5
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Quote:
Originally Posted by snostorm View Post
most lenses will show a bit of purple fringing and or CA, especially at the edges and corners, and usually it's more evident at larger apertures and with slightly out of focus areas
I've never seen it as bad as this though. My first thought was that the lens was dirty, and had a film over it.
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