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Old Oct 16, 2004, 9:36 PM   #1
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I've been seeing a lot of mention lately about purple fringing on the Canon A series cameras. I've been trying to educate myself on what contributes to this effect. Other than this factor I am pretty convinced to purchase an A series camera. I'm concerned that I've seen more talk about it on the A95, which is the most desiresble model to me for it's feature set. From the research I've done I can't see any fundamental reason why a 5 MP camera should be more prone to this problem. The problem seems to originate from the sensitivity of red & blue pixels of the CCD to IR & UV light combined with the physics of IR & UV light behaviour in lenses. My understanding is that a user can have some control by avoiding overexposure of bright area in a scene or using IR filters ( not exactly applicable to a point & shoot compact camera design though). Anyone have any insight into this issue in regards to the A95, relative to say the A75 or 85?

Also I wondering how the CCD designfor the A95 should affect image noise compared to the A75 or A85?

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Old Oct 17, 2004, 3:40 AM   #2
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Purple fringing or Chromatic Aberration (CA) is an issue with zoom lenses trying to align the light rays which at opposite ends of the light spectrum travel at different speeds; you've probably seen the experiment where white light passing through a prism creates a rainbow:
http://vtc.ngfl.gov.uk/uploads/image...nbow-52186.jpg

That's basically the issue here...what you are seeing is the same effect, but as the camera only has three colour receptors (red, green, blue) you see the extremes of the spectrum, red & blue to make purple:
http://astro.isi.edu/games/chromatic_fig02.gif

Now, the reason it's an issue with zoom lenses is they have a variable focal length which changes the point of focus...a prime lense with only one focal length wouldn't have this issue as the point of focus would be fixed.

It's not just an issue with digital cameras, film cameras and telescopes can also suffer from CA.

Here's a page which goes into more detail:
http://www.dpreview.com/learn/?/Glos...rations_01.htm

Now why the A95 suffers more? Canon didn't do a good job designing the optics, or they used the same optics designed for another sized CCD, and hoped it would be "good enough" for point&shoot consumers.
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Old Oct 17, 2004, 11:51 AM   #3
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I agree with the explanation of the physics behind chromatic aberation. I would have thought, however,that the same lense (with thesame light bending characteristics) in combination with a larger CCD (hence presumably larger light receptors)would tend to decrease this effect. This would be because larger receptors would be more likely to pick up the extremes of bent light before it contacted neighboring pixels.

I guess what I'mlooking for is a direct comparison sceneshot, which exhibits strong chromatic abberation, at the same exposure settings taken by both a A75/A85 and a A95.Then one couldsee if the A95 is worse or better.
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Old Oct 17, 2004, 6:56 PM   #4
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Looks like Steve did have ashot of tree twigs in the foreground and bright sky in the background which gave horrendous purple fringing with the A95. When taking the same scene with the A75the sun position and hencelighting is different (foreground branches are sunlit & not as dark) but there's no purple fringing at all. I'm not an expert in photography so I can't say if exposure settings are the same but they certainly look similar.

It is very concerning to me that a lower priced 3MP (A75) camera takes a cleaner picture than a 5 MP (A95)higher priced camera by the same manufaturer. Perhaps though I'm missing something in this comparison.

I suppose at least image noise would be lower with the larger CCD sensor on the A95.

:?

satwar wrote:

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I guess what I'mlooking for is a direct comparison sceneshot, which exhibits strong chromatic abberation, at the same exposure settings taken by both a A75/A85 and a A95.Then one couldsee if the A95 is worse or better.
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Old Oct 17, 2004, 10:38 PM   #5
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Mikefellh wrote:
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...a prime lense with only one focal length wouldn't have this issue as the point of focus would be fixed.....
Ah, but that is what differentiates "cheap" lenses from quality glass. Every lens will have CA--including "fixed-focus" lenses--unless lens elements are included to mitigate it. CA is affected by how dispersive (that is--how the index of refraction varies by the wavelength of ligh) the lens material is and how curved it is. My eyeglasses, being polycarbonate, have a high dispersivity--so much so that I have to double check if CA is from my glasses or from a lens! And they are, for certain, "fixed-focus"! If you follow all that--it becomes apparent why "mega-zoom" lenses have CA problems. In order to have such an extraordinary range of focus, a lens must incorporate elements which have high curvatures. This will lead to exceptional CA at both extremes of the lens--wide angle and zoom--but which should be tolerable mid-range where most normal shooting is done. Have you ever wondered why mega-zoom lenses aren't available for 35 mm film and dSLRs? 35 mm film and dSLR imagers are much larger than the "prosumer" models. A lens with a very large focal range (e.g. a 12x range) for such an imager would require elemens of such a degree of curavture as to be impractical--at least for now. I'll avoid the math at this point.....

In other words--CA is a fact of lenses. As technology marches on, it will improve, but no lens is without it (even if you don't normally notice it)--fixed focus or not. Zooms, especially ultra-zooms, are far more susceptible.


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