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Old Dec 2, 2006, 11:30 PM   #1
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I just bought a new Minolta 75-300mm 1:4.5~5.6D Len from B&H. I took couple pictures of the moon at 300mmtonight, but I discovered that the purple light around the moon at F5.6 1/500", not at F16 1/125", both ISO=200, manual focus, no post process, only crop.


Moon #1 at F5.6

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Old Dec 2, 2006, 11:31 PM   #2
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Moon #2 at F16


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Old Dec 3, 2006, 3:30 AM   #3
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The first moon picture may be too small to see the purple, I crop portion and make it larger here.
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Old Dec 3, 2006, 12:28 PM   #4
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It's mostly due to lens quality. Chromatic Aberrations (which usually take the form of Purple Fringing with Digital, probably due to sensor microlens design) in high contrast areas is common with many lenses, even very good ones (especially in out of focus areas).

Most lenses are not as sharp at wide open apertures. In addition, judging from user reviews and MTF Charts, this Minolta is softer on it's long end,

You were using it at it's longest focal length of 300mm (where it's softest), at f/5.6 (it's widest available aperture, where it's going to be softest), shooting a high contrast subject (brighter moon against a dark background). Your focus may have been off slightly too. So, yes, I would not be surprised to see some purple fringing with this type of image.


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Old Dec 3, 2006, 4:37 PM   #5
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and I thought this was proof that the moon has an atmosphere!

Jim's right, it's CA from the bright moon on a field of black.
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Old Dec 4, 2006, 6:05 AM   #6
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JimC wrote:
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It's mostly due to lens quality. Chromatic Aberrations (which usually take the form of Purple Fringing with Digital, probably due to sensor microlens design) in high contrast areas is common with many lenses, even very good ones (especially in out of focus areas). ...
I thought the issuewas called "blooming" and was caused by spillage of light from a brightly lit pixel to adjcent dark pixels. Easy enough to test - Chromatic Aberrations are worse near the edge of the lens while blooming shouldn't vary with possition. Just shoot the moon framing it near the corner instead of at the center. If it is a lens issue, there should be a variation from edge to center.
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Old Dec 4, 2006, 6:46 AM   #7
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I've seen the causes debated for years. The early conensus is that most of it was being caused by microlens design.

I mentioned blooming as a contributing factor in this recent thread about Purple Fringing, and noted that Kodak engineers associated it with the microlenses at one point:

http://www.stevesforums.com/forums/v...amp;forum_id=2

But, when I see it in a softer portion of a frame that's not really overexposed, and it goes away when stopping down (as it did in these examples), lens quality is a factor.

i've got some lenses that have far more than others.

I've also seen it referrred to as CA of the Microlenses. lol

According to Canon, coatings is even a factor. The G6 has significantly less Purple Fringing compared to the G5, and Canon indicates that lens coatings were changed to reduce it (which seems odd). I saw the impact of that debated a while back, too (with engineers saying coatings should not have anything to do with it, yet the proof is in the photos if you shoot side by side with them). The lens specs are the same between them and according to Canon, improved coatings reduced the purple fringing.

So, reflection/refraction due to coatings is an issue, too (and you will also see it from time to time when someone uses a cheap filter, impacting both purple fringing and flare).

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Old Dec 4, 2006, 10:13 AM   #8
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If you shoot the image in RAW, it's pretty easy to fix using the lens tab in the Adobe Bridge Raw conversion.
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Old Dec 4, 2006, 3:53 PM   #9
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JimC wrote:
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...
But, when I see it in a softer portion of a frame that's not really overexposed, and it goes away when stopping down (as it did in these examples), lens quality is a factor.
...
I agree - if the effect goes away when stopped down, the problem is almost certainly in the lens. If it were "blooming", I'd expect that the problem would increas when stopped down since the contrast would increase
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