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Old Dec 2, 2003, 11:03 AM   #1
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Default Purple Fringing



I have read comments about "purple fringing" and have seen evidence of it in a sample photo from the Oly C750. Can anyone explain what this is?

Is this a digital film problem, if so does a CMOS chip have the problem too?
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Old Dec 2, 2003, 4:05 PM   #2
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It's a lens issue, as different coloured wavelengths travel at different speeds that they don't reach the CCD at the same time...usually seen in severe contrasts. Ultra zoom cameras tend to suffer more as it's difficult to keep the wavelengths in line for the whole zoom range. BTW, regular film cameras can suffer from it too...it's not a digital or CCD issue.

More info here, http://www.dpreview.com/learn/Glossa...rations_01.htm
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Old Dec 2, 2003, 4:09 PM   #3
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Part of the problem is also due to CCD Blooming. This is where the charges in one photodiode spill over into adjacent pixels.

This tends to amplify Chromatic Aberrations problems.

If you look at some of the tests comparing the Canon S45 to the S50; or Canon G3 to the Canon G5; you'll see higher Chromatic Aberrations from the newer models.

The newer models use a 1/1.8" 5MP CCD; versus the older models that used a 4MP 1/1.8" CCD. Even though the CCD size, and lenses are the same between the older and newer models, Chromatic Aberrations are higher in the newer models -- most likely due to CCD Blooming from the denser sensors, with smaller pixel pitches (size of the photodiodes on the sensors). CCD Blooming tends to amplify CA problems.
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Old Dec 2, 2003, 9:30 PM   #4
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Thanks to both Mike and Jim for your info and reference.

I have taken thousands of 35 mm slides, hundreds of 2 1/4 by 2 1/4 transparencies, etc. and had never seen this fringing.

Is it possible that the CMOS type element in the Canon 300D or 10D is less susceptible to the blooming? Would this be a compelling argument for the extra $$$ as well as for the better lens?

Thanks,
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Old Dec 3, 2003, 1:41 AM   #5
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I am not an expert on the models you mention but the CCDs in them are quite somewhat bigger than the CCDs used in compact digicams so you can expect that blooming will be less.

Check out the reviews at www.dpreview.com and others that have specific tests that look into this effect.
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Old Dec 3, 2003, 3:14 AM   #6
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Mike has posted the links which show the effect. It's not difficult to find either. The first 10 shots I took with a Canon A80 in my garden, early morning, daylight white balance, grey overcast sky and with plenty of trees. Most tree branches were totally obscured by fringing. You know it's permanent by comparing the Blue and Green channels in PS.

The popular fix in PS is to match the fringe colour and make it grey. This only makes it less visible. I looked at both Nikon and Minolta who claim better lens systems. Although there are features about Minolta's S414 I didn't like, CA on identical shots was virtually absent and with good edge detail - what I'd expect from a 4 Mpix camera. If the Minolta lens had been in the A80 I'd have been a happy bunny! Here's the S414 lens spec:

4x Zoom 7.15 - 28.6 mm (35mm equivalent: 35 - 140 mm)
f/ 3.0 - f/ 3.6, 11 elements in 9 groups
(includes 1 AD glass element and 2 double-sided aspheric elements)
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Old Dec 3, 2003, 10:47 AM   #7
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No camera is immune to it -- especially in high contrast conditions, like tree limbs/leaves against a bright, white sky. Here's an example of one taken with a Minolta S414:

http://www.pbase.com/image/22234066

Sure, some are better than others, but any can exhibit this problem under some conditions -- especially in slighty overexposed areas of a photo (more common in the conditions the above photo was taken in), where CCD Blooming can also amplify any CA.
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Old Dec 4, 2003, 6:28 AM   #8
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Being a former OLY UZ shooter (I made the jump to the Cannon 300D), I had some success in correcting the dreaded purple fringing using Photo Shop Elements. This usually works in low light or backlit portions of a photo. Use one of the lasso tools to outline the area (or inverse the outline, which ever works best); then go to filters; select Noise; Use the Median slider bar to soften the selected area, usually a 1-3% adjustment is all that is needed. Most of the time this diminished or eliminated the purple fringing.
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Old Dec 4, 2003, 7:05 AM   #9
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Hey Mike, isn't that angles that you're referring too?
Quote:
It's a lens issue, as different coloured wavelengths travel at different speeds that they don't reach the CCD at the same time...
What's faster than the speed of light? Certainly not shutter speed... Different wavelenghts (ie colors) are bend differently as they travel through different mediums, ie glass/air/glass/air...etc such as they don't focus or line up on the same spot on the film/sensor plane, but IMO they strike at the same speed or least faster than the CCD that can capture them. :lol: :lol: :lol:
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Old Dec 5, 2003, 6:38 AM   #10
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NHL

Ever gave thought as to why light breaks when traveling through different mediums (air, glass, water, etc)? And why that is different for different wavelengths?

It has to do with the speed of light and that that speed is altered differently for different wave lengths in different media. Maybe you got confused by (special) relativity theory stating that the speed of light is a constant? that theory holds in a universe mostly made of vacuum like ours or a universe of glass. However the speed of light would be different for each of those universes.
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