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02-22-2012, 10:50 PM   #31
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QuoteOriginally posted by Lowell Goudge Quote
What I see is CA not purple fringing, so let's define the two, because they are different.
Indeed, they are not the same. They may look similar in some cases, but that is as far as it goes. PF is distinctive. I shot color film for several decades before getting my first digital camera (Canon G2) and was frankly dismayed by PF. I had never seen anything like it. When I got my K10D, I was shocked to see the same phenomenon with lenses that I had been using for years with film with no problem.

A point of departure might be to consider the new Fuji sensor on the X-Pro 1. One of the rationals of the design was to eliminate PF.


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02-22-2012, 10:50 PM - 1 Like   #32
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QuoteOriginally posted by vrphoto Quote
Thanks, mikeSF. Lens correction doesn't remove purple fringing for me, but reducing Purple saturation does in both Lightroom and Photoshop, it's a pity about the effect on the purple tones in other parts of the picture . I hope that LR4 will make this a bit easier.

when you have legit purple tones in your pic you wish to preserve, simply apply the purple slider reduction to a duplicate layer and then paint in the original layer over just the areas you need purple...does that make sense? i know it's a few more steps, but very useful.


I only need this and other more involved methods less than 5% of the time as I get excellent results using the CA sliders in ACR.

Last edited by mikeSF; 02-23-2012 at 08:52 AM.
02-22-2012, 10:54 PM   #33
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QuoteOriginally posted by Lowell Goudge Quote
True PF from sensor blooming is quite different than CA and is only present at extreme bright to dark transitions. cA is there all the time.
Yes, CA is there all the time and degrades all areas of the image equally, not just the places where you see it. It is a huge issue for B&W film work as a result.


Steve
02-22-2012, 10:57 PM   #34
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QuoteOriginally posted by vrphoto Quote
Would you mind pointing me in the direction of the Photoshop trick?
Thanks
There are CA correction sliders in ACR and Lightroom. To correct for PF in Lightroom, there is a defringe feature. You can also take the "dropper" in the saturation slider section, select the area of PF and drag the "dropper" down to selectively desaturate that color mix from the image.


Steve

02-22-2012, 10:59 PM   #35
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QuoteOriginally posted by Laurentiu Cristofor Quote
the definition is quite generic and the sample provided is not one where purple surrounds a bright spot - it is closer to what wlachan posted.
Definitely not always associated with a bright spot and definitely not as confined as CA. Blooming or Webbing are the two terms that come to mind.


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02-22-2012, 11:07 PM   #36
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QuoteOriginally posted by Anvh Quote
Very nice artiacle about CA and he also talks about PF
Chromatic aberrations

What most call PF is actually longitudinal chromatic aberration.

And it isn't a sensor thing, at least not for the most part.
If it was then tell me why it does not show up with all the lenses?
An excellent article and one of the classics on the Web in regards to showing CA. OTOH, loCA as illustrated is not the same as classic PF. The classic setup to demonstrate CA is a sharp boundary between light and dark with a series of images stepping through the focus planes. The boundary allow for easy visualization of the focus aberration that is present throughout the frame. The classic setup for PF is to have a curved chrome plated metal surface in full sunlight, somewhat off-center (think boat or pool railing). That chrome will have purple and blue wildness while even when the light/dark boundaries in the same frame are clean and CA free and is present at the plane of focus.

As for why it is more prevalent with some lenses...the last time I looked, it was thought that angle of incidence to the sensor might be the issue along with the size of the rear element. My big question is why, if it a lens thing, is there no mention of PF in the photo magazines, books, and reviews prior to the digital era? I have been active in photography since the late 1960's and never heard of PF until I was shopping for my Canon G2.

If any of you doubters is particularly brave, you could always hazard an inquiry on APUG about PF.


Steve

Last edited by stevebrot; 02-22-2012 at 11:16 PM.
02-23-2012, 12:03 AM   #37
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QuoteOriginally posted by mikeSF Quote
when you have legit purple tones in your pic you wish to preserve, simply apply the purple slider reduction to a duplicate layer and then paint in the original layer over just the areas you need purple...does that make sense? i know it's a few more steps, but very useful.
mikeSF, thanks again, I understand. It was quite a shock to see these coloured edges coming from such a lovely lens and I was hoping for a quick fix, but your method will do.

stevebrot, CA correction doesn't work for me at all in the pic I loaded above, and I've never been able to see any change as a result of using the "defringe" in Lightroom. But I'm ok that PS layers will do the trick.
02-23-2012, 03:04 AM   #38
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QuoteOriginally posted by vrphoto Quote
Happy to call it whatever - sensor blooming sounds ok to me. It's the removal that's the hard thing to do

If my first attempt at attaching a picture works , you'll see that "purple blooming" applies in 2 ways, where if I remove the unwanted purple edges around the icing, the agapanthus petals on the left are also affected (not that it bothers me in this happy-snappy) ... fyi, the lighting is indirect daylight coming from the south (Australian orientation), so not really backlit.
Quick fix in Photoshop: 30 seconds(may be less depending on how much coffee you have in you ), and 5-7 mins. if you get picky about it on the icing(PF). I generally try to limit the edits to the apparent areas. ie. those that catch the eye. Though it really comes down to a case by case basis depending on the intent of the photograph(print, visual etc).

Having said that, the best control method I've found(to date) is to shoot in RAW and approach the issue from a multitude of angles with specified software. For example; ACR and LR can deal with CA to some degree, though their correction measures are mostly limited to registration issues and the color registration(RGB) produced by the optics. Whereas it usually does very little for PF which runs along the lines of blooming. <-Which btw. is a limitation in itself regardless of the color correction given that it produces a(sort-of) haze around edges after the fact which can only be corrected through intrusive editing(edge reconstruction). <- to my knowledge that is.

Anyways, my solution is to shoot RAW, and run the file through RAW Therapee. Which has a CA module(similar to ACR and LR), and more importantly... a fringe correction module which allows to address Fringing(of all channels, RGB) which is quite effective. However... in certain cases, where color fidelity is of the utmost importance, it is often necessary to render two working files: 1) a PF corrected one and 2) a none corrected one in PS so as to mask-in and out the edges and areas that need correction respectively.

Aside from that, the process is usually topped-off with the old 1 min. PS army trick. That is the Gaussian blur /color effects method. Which is about as old as I am(pretty old). And surprisingly enough, quite obscure in our day despite its wide spread use in the industry. Which I had no idea why, but I'm guessing the pro's don't like sharing their trade secrets:






PS. there are times when PF is present in a scene in a very significant way(throughout the scene). Which can often be seen with images containing lots of chrome etc. And in such cases, I've found that corrections are best left to dedicated software such as: Helicon Filter which has a dedicated PF/fringing correction module. However, in this particular case, has the ability to target fringing by color rather than by highlight detection.

Needless to say, when all is said and done, CA/PF corrections are not always easy to deal with. Though in my own experiences are always possible to fix. And so I guess it really comes down to our own level of commitment toward the issue.

Hope this helps


Last edited by JohnBee; 02-23-2012 at 03:40 AM.
02-23-2012, 02:32 PM   #39
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QuoteOriginally posted by Lowell Goudge Quote
Except that there is no need to redefine CA as purple fringing.
Nobody tried that. I just pointed out that most people will see purple CA and will call it purple fringing. It's also not clear that all PF issues are cause by blooming. The image in the wikipedia article looks like neither blooming nor regular CA - how do we call that? PF is a fuzzy term.

QuoteOriginally posted by vrphoto Quote
Happy to call it whatever - sensor blooming sounds ok to me. It's the removal that's the hard thing to do
Well, it matters what it is if you want to avoid it. I get purple blooming from some old lenses. I don't know how to remove it - even if you desaturate the purple, you cannot recover the detail behind the bloom area. So I just avoid using those old lenses in scenarios where they're producing such effects - sometimes I avoid using them entirely. With new lenses, I don't get much fringing at all, so I don't need to cleanup anything.
02-23-2012, 02:35 PM   #40
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QuoteOriginally posted by stevebrot Quote
My big question is why, if it a lens thing, is there no mention of PF in the photo magazines, books, and reviews prior to the digital era?
Could it be that no one was printing large enough to be able to pixel peep and be horrified by the purple? With digital, we all started zooming into images and exploring all the exquisite detail, then suddenly we noticed a boatload of purple. The horror! The horror!
02-23-2012, 05:32 PM   #41
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QuoteOriginally posted by stevebrot Quote
As for why it is more prevalent with some lenses...the last time I looked, it was thought that angle of incidence to the sensor might be the issue along with the size of the rear element. My big question is why, if it a lens thing, is there no mention of PF in the photo magazines, books, and reviews prior to the digital era? I have been active in photography since the late 1960's and never heard of PF until I was shopping for my Canon G2.

Steve
Steve its so simple, pure pimple fringing is a digital issue, where as CA is a lens issue and was discussed in the late 1970's with the introduction of the first aprochromatic lenses and the use of rare element glass. If you search for photos I have posted you will find a red shouldered hawk in flight shot with. Kodacolor 400 using my K300/4 and 1.7x AF TC. There is plenty of green and purple CA artifacts evident. CA has always been there. But you can't produce sensor bloom by scanning film, I've tried. It is a DSLRs only issue. As for pixel peeping and viewing enlargement maybe print film was limited in typical enlargement, but I viewed slides on a 60 inch wide screen that is about equal to 1:1 pixel mapping on my 22 inch 16x9 format monitor with K5D images. Who ever said we didn't look that close at our film shots?
02-23-2012, 06:00 PM   #42
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The way I see it, lens CA and sensor blooming are 2 different issues. PF is a generic term which can be associated to either one, but somehow being firmly associated to sensor blooming by some. I have no idea why.
02-23-2012, 09:26 PM   #43
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QuoteOriginally posted by Laurentiu Cristofor Quote
Could it be that no one was printing large enough to be able to pixel peep and be horrified by the purple? With digital, we all started zooming into images and exploring all the exquisite detail, then suddenly we noticed a boatload of purple. The horror! The horror!
Surely you jest!


...at least I hope you are. I can't imagine that photog-types suddenly became anal-retentive with the onset of digital! Honestly though. I have gone back through my slides looking for PF and can't find any when enlarged to wall size (I still own a projector).


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02-23-2012, 09:28 PM   #44
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QuoteOriginally posted by wlachan Quote
The way I see it, lens CA and sensor blooming are 2 different issues. PF is a generic term which can be associated to either one, but somehow being firmly associated to sensor blooming by some. I have no idea why.
As one of those who would prefer purple fringing to be only addressing sensor bloom I will reiterate my earlier statements. Fringing of any color due to CA is and should always be called CA and not purple fringing. Call it what it is don't confuse the issue by using imprecise or vague terms when a correct accurate and precise term exisis. If you want a good example of how messed up things get, just look at the "crop factor" and all the confusion it has caused. By dumbing down the terms used we throw people into confusion when there is no need
02-23-2012, 09:42 PM   #45
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If we are going to be exact then we should call it axial chromatic aberration.
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