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Old Jan 3, 2007, 9:38 PM   #1
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There's a "test shot" that I like to take with each new digital camera that I get. I call it "the purple fringing torture test". My very first 1 megapixel Olympus point-and-shoot (years ago) failed horribly at this test, and later cameras I've owned have done much better.

I take the shot at the nuclear power plant where I work. The high intensity lighting in the ceiling of the turbine building tends to cause purple fringing on digital cameras.

I was shocked to see what happened when I took the shot with my new Canon Digital Rebel XTi camera. I haven't seen this much purple fringing since I took this same shot years ago with that 1 megapixel Olympus point-and-shoot.

I have to think that the Digital Rebel XTi itself is sophisticated enough that the camera is probably not to blame. My prime suspect is the cheap 18-55mm kit lens that I was using at the time. My guess is that if I took the same shot with my "L" series lens, I wouldn't get the purple fringing. (I will definitely have to try it sometime.)

My question is... do you think my theory is correct? Could a cheap lens be to blame for purple fringing? Do you think the same shot with my 70-200mm "L" lens will turn out better?

I am attaching the photographic evidence, so you can see that I'm not exagerating about how badly the XTi with the kit lens failed the purple fringing test!


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Old Jan 3, 2007, 9:39 PM   #2
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Here's a crop of that last photo, at 100% of the original.
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Old Jan 3, 2007, 9:53 PM   #3
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By way of comparison, here's the same shot from two and a half years ago... taken with a Panasonic DMC-FZ20


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Old Jan 3, 2007, 9:54 PM   #4
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And the shot from the DMC-FZ20, cropped and viewed at 100%...
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Old Jan 3, 2007, 10:43 PM   #5
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That purple fringing is chromatic aberration. I think it is when the different colours of light don't hit all on the same spot on the sensor. It is a function of the lens used. Better quality lenses would have better control of CA.





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Old Jan 4, 2007, 1:55 AM   #6
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All lenses display chromatic aberration to some degree.

In fact the cornea of the eye does too! If you pay attention you can see it sometimes, but the brain is very good at correcting for the deficiencies of the lens in software.

http://en.wikipedia.org/wiki/Chromatic_aberration


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Old Jan 4, 2007, 7:52 AM   #7
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it's the lens that produces the fringing not the camera or sensor or processor
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