Lindolfi Posted June 19, 2011 Share #41 Posted June 19, 2011 Advertisement (gone after registration) Just a single off-topic remark: a good and productive discussion as a tool to get closer to the most reliable explanation of anything we still don't fully understand in a group effort, is characterised by avoiding qualifications of each other as discussion partners and by only handling the discussion subject at hand. That is a long sentence that can be summarised by the advise : "play the ball, not the man" Link to post Share on other sites More sharing options...
Advertisement Posted June 19, 2011 Posted June 19, 2011 Hi Lindolfi, Take a look here Purple Reflection. I'm sure you'll find what you were looking for!
Rick Posted June 19, 2011 Share #42 Posted June 19, 2011 denoir, in your post of the screws (#35) I have to ask you why there is no shift from purple to green as the PF on the screws is not equally in the plane of focus? Second, in the post of the chart, why is the color reversed? Also, the photos taken with different lenses may demonstrate an effect of LoCA correction of these different lenses, but aren't there a host of other optical differences between these lenses, such as exit pupil size and distance? I like to follow you, but the simplistic approach doesn't seem to explain all of this. I think you have part of it, but no matter how many times you say the same thing, it still doesn't completely explain PF. Link to post Share on other sites More sharing options...
denoir Posted June 19, 2011 Share #43 Posted June 19, 2011 denoir, in your post of the screws (#35) I have to ask you why there is no shift from purple to green as the PF on the screws is not equally in the plane of focus? There is actually - it's only behind and not included in the crop. That is the case in that sample. Where the green will be visible will depend a lot on how well the lens is corrected. Green is much easier to correct for than violet so usually in high end lenses you won't see any green fringing near the focal plane. There may still be in the OOF areas though as the correction is a function of among other things distance to the focal plane. Here's an example of exactly that with the 75 Cron APO. Welcome, dear visitor! As registered member you'd see an image here… Simply register for free here – We are always happy to welcome new members! I had to go to 200% crop for the PF to be visible in this case: I'm sure you had no trouble spotting the green fringing in the first shot, but here's a 100% crop: Second, in the post of the chart, why is the color reversed? It's in the usual way - pruple in the front, green in the back. Also, the photos taken with different lenses may demonstrate an effect of LoCA correction of these different lenses, but aren't there a host of other optical differences between these lenses, such as exit pupil size and distance? Yes, there are of course other differences - but you can't get around it. And there is no established connection between exit pupil size and PF or any type of CA. I like to follow you, but the simplistic approach doesn't seem to explain all of this. I think you have part of it, but no matter how many times you say the same thing, it still doesn't completely explain PF. I've tried to point out that there is nothing to explain. Chromatic aberrations are not a mystery but very well known fact. Lloyd Chambers in his "Making Sharp Images" guide has a chapter called "Axial Crhomatic Aberration (Purple Fringing)": http://diglloyd.com/prem/prot/MSI/Optical-aca.html It requires a subscription. I won't copy paste the article here, but here's a quote The cause of violet fringing is almost always a failure to correct chromatic aberrations in the 400 nanometer range (violet). Or if you don't like or trust Chambers and don't have a subscription, how about some Erwin Puts? He even precisely shows with a correction graph and sample images how an S2 75 Summarit lens produces purple fringing: Chromatic aberrations The list is endless. That PF is somehow a mystery is a myth that has been perpetuated on internet forums for years, but anybody that knows optics also knows that there is no mystery. It's just good old LoCA, nothing more and nothing less. There are books upon books that will tell you so. I really don't understand why this myth of PF being something special is being perpetuated when it is a well known and well documented phenomenon. Link to post Share on other sites Simply register for free here – We are always happy to welcome new members! I had to go to 200% crop for the PF to be visible in this case: I'm sure you had no trouble spotting the green fringing in the first shot, but here's a 100% crop: It's in the usual way - pruple in the front, green in the back. Yes, there are of course other differences - but you can't get around it. And there is no established connection between exit pupil size and PF or any type of CA. I've tried to point out that there is nothing to explain. Chromatic aberrations are not a mystery but very well known fact. Lloyd Chambers in his "Making Sharp Images" guide has a chapter called "Axial Crhomatic Aberration (Purple Fringing)": http://diglloyd.com/prem/prot/MSI/Optical-aca.html It requires a subscription. I won't copy paste the article here, but here's a quote Or if you don't like or trust Chambers and don't have a subscription, how about some Erwin Puts? He even precisely shows with a correction graph and sample images how an S2 75 Summarit lens produces purple fringing: Chromatic aberrations The list is endless. That PF is somehow a mystery is a myth that has been perpetuated on internet forums for years, but anybody that knows optics also knows that there is no mystery. It's just good old LoCA, nothing more and nothing less. There are books upon books that will tell you so. I really don't understand why this myth of PF being something special is being perpetuated when it is a well known and well documented phenomenon. ' data-webShareUrl='https://www.l-camera-forum.com/topic/154554-purple-reflection/?do=findComment&comment=1707171'>More sharing options...
Rick Posted June 19, 2011 Share #44 Posted June 19, 2011 Ok, thanks for the answers. 1. Ok, I'll believe you that the green is there. Why is it that it is much easier and more common to find purple examples? 2. I thought it through and I was thinking backwords on the chart. The purple would be in front and the green behind the plane. Green is bent less and would be behind the plane. 3. I like the duck example. I'll have to read your articles that you suggested. Edit: Having read the articles I can see that the purple around the green focus could be a lot more prominent and would be picked up more especially if the green spectrum is chosen as the focus point. Link to post Share on other sites More sharing options...
Lindolfi Posted June 19, 2011 Share #45 Posted June 19, 2011 Ok, thanks for the answers. 1. Ok, I'll believe you that the green is there. Why is it that it is much easier and more common to find purple examples? Look at the second panel in figure 5 of this page http://toothwalker.org/optics/chromatic.html, reading 'achromat': the colors red and blue (together forming purple) are both further removed from the average than green, due to the parabolic shape of the curve. Further removed from zero means more spread around the highlights in the image due to longitudinal chromatic aberration. On top of that: to the eye, green is most important for establishing what is in focus, so the zero line would be closer to the green component of the image. Link to post Share on other sites More sharing options...
Washington Posted June 20, 2011 Share #46 Posted June 20, 2011 Knock out the purple in Lightroom with the purple saturation and purple density sliders. Link to post Share on other sites More sharing options...
Beyder28 Posted June 20, 2011 Author Share #47 Posted June 20, 2011 Advertisement (gone after registration) Absolutely. It is the sensor that has to deal with all that light in the end and very often in the PF case one is pushing its boundaries. I definitely agree that the sensor contributes to enhancing the effect. In the case of the M9, I'm also pretty sure that the lack of an AA filter helps accentuate the edges of the fringes, making them more apparent. What I've just been saying is that CA is the cause of it. Anyway in practice the solution to getting rid of PF is to a) stop down the lens use a lens with better corrections or c) remove in post processing. Hello, I'm the OP and I appreciate all the great info that is coming out of this debate. That was one of the first pics I took with the 50mm cron (current version) which i just baought used. However, I have not seen it with the other 150 or so shots I took with it. Weather it's PF or CA, could/should this happen with this lens? Link to post Share on other sites More sharing options...
Lindolfi Posted June 20, 2011 Share #48 Posted June 20, 2011 Well, Beyder28, yes it could happen as you have shown. In some lens designs more often than in others. Subjects with small higlights are prone to it. As to the question whether it should happen: only if you have some artistic preference for purple fringes. Lens errors like vignetting, spherical aberration and coma may give a certain desirable signature to images, but chromatic errors usually do not. In the example you gave, it is not an improvement I think. If you want to use the image, it can be handled in postprocessing, as Washington suggested. Link to post Share on other sites More sharing options...
jaapv Posted June 20, 2011 Share #49 Posted June 20, 2011 I am not a particular fan of Wikipedia, but I think this particular summry is spot-on, and it does show that everybody is right in this thread: The term "purple fringing" is commonly used in photography, although not all purple fringing can be attributed to chromatic aberration. Similar colored fringing around highlights may also be caused by lens flare. Colored fringing around highlights or dark regions may be due to the receptors[clarification needed] for different colors having differing dynamic range or sensitivity -- therefore preserving detail in one or two color channels, while "blowing out" or failing to register, in the other channel or channels. On digital cameras, the particular demosaicing algorithm is likely to affect the apparent degree of this problem. Another cause of this fringing is chromatic aberration in the very small microlenses used to collect more light for each CCD pixel; since these lenses are tuned to correctly focus green light, the incorrect focusing of red and blue results in purple fringing around highlights. This is a uniform problem across the frame, and is more of a problem in CCD's with a very small pixel pitch such as those used in compact cameras. Some cameras, such as the Panasonic Lumix series and newer Nikon DSLRs, feature a processing step specifically designed to remove it. Link to post Share on other sites More sharing options...
Lindolfi Posted June 20, 2011 Share #50 Posted June 20, 2011 Thanks Jaap for the quote. Lens flare is a phenomenon of spreading of light in the image from the light source and has the colour of the light source, so not purple. Highlights can be seen as indirect lightsources, but a spreading of light around the image of the lightsource is caused by defocus, sperical aberration or coma (all giving no colour dispersion), or by chromatic aberration (the only one that does give colour dispersion). So I feel that the Wikipedia article you lifted the quote from, may need a revision. Link to post Share on other sites More sharing options...
jaapv Posted June 20, 2011 Share #51 Posted June 20, 2011 Well, what is stopping you? The quote I posted emphasizes the mulitcausal nature of the problem, and that is unchanged I think. Either-or discussions are a bit of nonsense in cases like this imo (guilty by my first post in this thread:o - I still think on that particular image the main cause is not CA, but that is no more than an opinion.) Link to post Share on other sites More sharing options...
denoir Posted June 20, 2011 Share #52 Posted June 20, 2011 I am not a particular fan of Wikipedia, but I think this particular summry is spot-on, and it does show that everybody is right in this thread: *sigh* No Jaap, as mentioned early in the this thread the Wikipedia "article" is wrong. The purple fringing section in the CA article and the PF article manage to contradict each other and at the same time both be wrong. As you can see no references have been provided. I may rewrite both when I get a bit of time and then it will be backed up by proper references. I'm really curious though what it is that makes PF an article of faith. I mean this is getting perfectly ridiculous. It's clear that I can give you number of expert references that tell that it's CA and nothing more and that it will be just ignored by some. Link to post Share on other sites More sharing options...
jaapv Posted June 20, 2011 Share #53 Posted June 20, 2011 I may well sigh too. I am not saying purple fringing in general is not CA - it often is. I am just saying there are more possible causes. For all you know that necklace may be lined with Heliotropes. Link to post Share on other sites More sharing options...
Lindolfi Posted June 20, 2011 Share #54 Posted June 20, 2011 The quote I posted emphasizes the mulitcausal nature of the problem Just to make things clearer: there is an important difference between "multicausality" and "serial causality" If I push a vase from a roof, and the vase is broken on the floor, the primal cause is the force from my hand, gravity takes over as the second cause and the collision with the ground is the third cause, all three necessary for breaking the vase. If you and I would both push together, the forces from both our hands would be called "multicausal" The claim in this thread is that CA is the cause and the sensor properties take over in enhancing the visibility, so no multicausality in that sense but serial causality. Link to post Share on other sites More sharing options...
jaapv Posted June 20, 2011 Share #55 Posted June 20, 2011 Ok, I know the difference between serial and multi, although the terms get confused for killers;) - but why can I see it on my 135 Apo Telyt then? A lens that is apochromatically corrected to the third spectrum? That would be a vase breaking without falling. Link to post Share on other sites More sharing options...
stickan1 Posted June 20, 2011 Share #56 Posted June 20, 2011 I have followed this discussion with interest, but also been annoyed with the lack of real communication at times. I think that Denoir has got most of the physics correct, but some of the problems is not about physics, but language. It seems that the word "purple fringing" is used for a rather wide array of phenomena in this tread, although it should from a more scientific point be used for optical disturbances in the lens caused by CA. This happens often. I got a PhD in applied physics, but for the last 10-15 years I have frequently worked together with persons who have very different backgrounds and come to learn and accept that words means different things for different persons. They may have an incomplete or incorrect understanding of the phenomena discussed, but if you want to communicate with them you have to understand how they think and adapt. Link to post Share on other sites More sharing options...
01af Posted June 20, 2011 Share #57 Posted June 20, 2011 I may rewrite both when I get a bit of time and then it will be backed up by proper references. Oh no, please don't! You're not the right person to do it. There are enough articles already on Wikipedia written by self-assigned "experts" like you I'm really curious though what it is that makes PF an article of faith. I mean this is getting perfectly ridiculous. Do now know Occam's Razor? It a principle of logic that says, of several competing theories that try to explain an observed phenomenon, the simplest that can plausibly explain all observations without contradictions most likely is the correct one. Your explanation ("purple fringing is longitudinal chromatic aberration") however is too simple. It can explain a few things about purple fringing but not all. So according to most fundamental logic, it must be wrong ... or at least incomplete. Unfortunately this insight does not automatically lead to the correct explanation—but we know we must look elsewhere. It's clear that I can give you number of expert references that tell that it's chromatic aberration ... So by definition, an expert is a person that agrees with your opinion!? Ad Hominem removed Link to post Share on other sites More sharing options...
Lindolfi Posted June 20, 2011 Share #58 Posted June 20, 2011 ?..but why can I see it on my 135 Apo Telyt then? A lens that is apochromatically corrected to the third spectrum?. Look at the third panel of figure 5 of this page http://toothwalker.org/optics/chromatic.html, reading "apochromatic". See how the blue deviates strongly from the zero line, although there is a correction at three wavelengths. Link to post Share on other sites More sharing options...
jaapv Posted June 20, 2011 Share #59 Posted June 20, 2011 I would like to draw attention to this post by MJH, where he states that purple fringing can be induced on a sensor by laser light, which means without any CA present: http://www.l-camera-forum.com/leica-forum/leica-m9-forum/107052-m9-50-lux-asph-too-much.html#post1132055 Link to post Share on other sites More sharing options...
Lindolfi Posted June 20, 2011 Share #60 Posted June 20, 2011 I would like to draw attention to this post by MJH, where he states that purple fringing can be induced on a sensor by laser light, which means without any CA present: http://www.l-camera-forum.com/leica-forum/leica-m9-forum/107052-m9-50-lux-asph-too-much.html#post1132055 Good, that is a statement that can be tested! A green laser with a wavelength of 532 nm is not dispersed by optics into different colours so if a sensor produces purple fringing out of 532 nm waves, there is multicausality. I'll do the experiment and report back. Link to post Share on other sites More sharing options...
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