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Author Topic: Purple fringing  (Read 3306 times)

Dale_Cotton

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Purple fringing
« on: September 05, 2002, 08:22:49 am »

[font color=\'#000000\']Fringing is chromatic aberration by another name. This is a lens aberration caused by the glass in the lens having a different index of refraction for the different wavelengths (colours) of light. IOW, the lens acts like a prism separating white light into colours. Lens designers can compensate for this effect to re-converge the colours to some degree but there are trade-offs involved. It would seem that correcting chromatic aberrations involves unacceptable trade-offs (perhaps to sharpness) in these small digicam zooms, because they nearly all have this problem. If you look at the www.dpreview.com review of any digicam you will find a test showing the degree of this problem for that camera.

To correct the problem my current technique is to make a selection that includes the fringing area, open the Saturation control, choose the appropriate colour channel, such as red or magenta, desaturate, then narrow the colour selection as much as possible using the colour bar at the bottom. If the fringing, when desaturated, is lighter or darker than it should be, I then adjust lightness accordingly.[/font]
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Kevin McCauley

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Purple fringing
« Reply #1 on: September 05, 2002, 02:10:37 pm »

[font color=\'#000000\']Dale- Thanks a lot for all the info. I have never heard chromatic aberration mentioned when referring to purple fringing, so that's news to me. I'll try your post-processing technique for fixing the problem. I've used similar methods with decent results, so I would expect the same here. Thanks again.

     Kevin[/font]
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McKev25

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Purple fringing
« Reply #2 on: September 04, 2002, 01:49:01 pm »

[font color=\'#000000\']I have an Olympus C-3040 that I'm very happy with. Sure, it has its problems, like most cameras do, but overall I think it is an excellent camera. One thing I have noticed is that in a few pictures, there is fairly obvious fringing. In one picture I took, there was a terrible purple fringe on some leaves in the direction facing the sun, but it was late in the day, so I'm not sure why that happened. More common, though, is a green fringe. It seems to occur in very sunny conditions, which leads me to believe it has to do with CCD blooming.

It doesn't seem that anyone has come up with a definite reason for this problem. I would guess that is does have something to do with blooming/UV contamination/highlights, but I haven't been able to predict when it will occur.

Does anyone have any thoughts on this, and are there any quick fixes for it in an image editing program?

Thanks in advance for your help :)[/font]
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Dale_Cotton

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Purple fringing
« Reply #3 on: September 05, 2002, 08:31:55 am »

[font color=\'#000000\']"but I haven't been able to predict when it will occur."

It occurs at the juncture of a dark area and a very bright area. For example the edge of a building or branch against a bright sky. It also tends to occur more toward the perimeter of the lens than the centre.

My understanding is that the brightness of the sky portion of an image is fringing throughout, but within that region the colours re-mix. It is only at a boundary with a dark border that the fringe colour is unaffected by anything of similar brightness.[/font]
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