[wim]

[quote name='Brightcolours' timestamp='1308165580' post='9266']

Wim, La CA usually is green and magenta, or red and green-blue, the yellow and blue variation is relatively rare. [/quote]

Funnily enough, while it often is green and magenta (Sigma lenses f.e.), officially it IS yellow and blue (that is how it was defined originally), and with Nikkors it is almost always yellow and blue.

LaCA affects the whole image, except the center of the image. The further you go to the edge, the stronger the LaCA will get. It messes up the image's contrast and sharpness, and in contrasty transitions you will see on one side of a darker subject magenta, on the other side green. Or, yellow and blue. Or red and blue-green. The colour depends on which part of the spectrum gets "bent" more or less than the rest of the spectrum, so which colour image has a larger or smaller projection than the rest of the spectrum. If red is the offending part of the spectrum, you will see red and blue-green edges. If blue is the "problematic" part, you will see blue and yellow, And if green is the offending colour, you will see magenta and green.

I am assuming you mean Lateral CA with La CA. I do know it gets worse towards the outside of the image, I was trying to keep things relatively simple. It does occur most in the in focus area however, purely because the difference between the (two) planes that it affects isn't that great, and because OOF blur causes it to disappear rapidly. Furthermore, you often only see 1 colour of the lateral CAs most pronounced, unless you shoot black and white squares on a flat test sheet plan parallel to the sensor and optical plane, or trees with leaves and enough sky area beteween them towards the corners of a frame. Apart from that, the picture displayed here is only a crop as far as I can see.



Also, if you really want to explain this, why not talk about the sagittal and tangential curves not coinciding which causes this?



The real reason for lateral CA is that the FLs for the different colours may not be the same, and although they may be sharp in the same plane, they are slightly offset as a result. This causes the fringing, and is also the reason why it is stronger towards the edges of the frame than it is towards the centre. And it also is the reason why it is most obvious in the area of focus, or rather within DoF.

CA, usually, is correctable, if the optics show a nice and gradual CA shift. It also depends on the "purity" of the offending part of the spectrum, because the software I know only corrects for the different colour channels (R, G or B ). The correcting works by making the image or either the red, blue or green channel larger or smaller, till the size corresponds to the other 2 channels.

Selective desaturation IOW.

There is also software that does not correct, but rather masks, CA by desaturating just the coloured edges. Most in-camera "CA-correction" works that way. This is a less desirable method.

I guess that wasn't directed at me.



BTW, no need to explain CAs to me, and neither was I asking for any comments on my post. I added my post here for 3 simple reasons:

1) Not all CAs clearly to be seen in the picture were covered.

2) Nobody explained exactly how these interacted in the picture and which these were (in discussions interactions between optical and other aberrations often get ignored).

3) Nobody indicated what a real quick solution to the problem was.



In short, I posted in reply to the OP, as an addition to what was said here, and based on the part photograph shown. And that's all.



Regards, Wim

[Brightcolours]

[quote name='wim' timestamp='1308171429' post='9268']

Selective desaturation IOW.

[/quote]

No, NOT selective desaturation. Shrinking one of the colour channels, or expanding it, will not desaturate anything. Just make all 3 channels the same size again, which will mean the coloured edges will disappear, the CA induced sharpness will disappear and the contrast will recover.



You shrink or expand it from the image center.



The one you think was not directed at you, that one is about desaturating of colours, which is the less desirable way.



About you not needing an explanation, what you wrote about the CA needed some correcting. There is no La CA to be seen, not the blue part of the spectrum of the nose at least. All that is just PF. CA would show a totally different, and directional, character. The upper part of the instrument, if it was CA, would show on one side, seen from the center of the image, blue, and on the other side yellow lines around the dark parts. On the sides that are not on the direction towards the center of there image there would be no CA.

That is just the nature of LaCA. What we see is PF and some LoCA.



The colour of CA needed further explanation too, as yellow/blue is quite rare.



And yes, you are correct in understanding that with LaCA I am referring to Lateral Chromatic Aberration, as opposed to LoCA (longitudinal Chromatic Aberration)

[wim]

[quote name='Brightcolours' timestamp='1308172389' post='9269']

No, NOT selective desaturation. Shrinking one of the colour channels, or expanding it, will not desaturate anything. Just make all 3 channels the same size again, which will mean the coloured edges will disappear, the CA induced sharpness will disappear and the contrast will recover.



You shrink or expand it from the image center.[/quote]

If you mean less desaturation in the centre and more towards the edges: that is what I mean by selective desaturation.

The one you think was not directed at you, that one is about desaturating of colours, which is the less desirable way.



About you not needing an explanation, what you wrote about the CA needed some correcting. There is no La CA to be seen, not the blue part of the spectrum of the nose at least. All that is just PF. CA would show a totally different, and directional, character. The upper part of the instrument, if it was CA, would show on one side, seen from the center of the image, blue, and on the other side yellow lines around the dark parts. On the sides that are not on the direction towards the center of there image there would be no CA.

That is just the nature of LaCA. What [color="#ff0000"]we[/color] see is PF and some LoCA.

Well, let's just agree to disagree in that case. What [color="#ff0000"]you[/color] see is PF and some LoCa, what[color="#ff0000"] I[/color] see is PF, LoCa, and lateral CA. IOW, WE do not see the same things, IOW, don't use WE in this case unless you are of royal descent and head of state. I am not in the group with the same opinion here, so please don't say "we".

The colour of CA needed further explanation too, as yellow/blue is quite rare.

As I mentioned: not with Nikkor lenses, and as I mentioned, it was defined with yellow and blue; I was taught so about 35 years ago that CA was defined as yellow/blue artefacts.

And yes, you are correct in understanding that with LaCA I am referring to Lateral Chromatic Aberration, as opposed to LoCA (longitudinal Chromatic Aberration)

Regards, Wim



P.S.: I am out of here, I don't have time for these discussions. As mentioned, I only wanted to add some hopefully useful info to this topic. You don't agree with me, but I didn't really expect anything else.



Cheerio.

[Brightcolours]

[quote name='wim' timestamp='1308178761' post='9270']

If you mean less desaturation in the centre and more towards the edges: that is what I mean by selective desaturation.



Well, let's just agree to disagree in that case. What [color="#ff0000"]you[/color] see is PF and some LoCa, what[color="#ff0000"] I[/color] see is PF, LoCa, and lateral CA. IOW, WE do not see the same things, IOW, don't use WE in this case unless you are of royal descent and head of state. I am not in the group with the same opinion here, so please don't say "we".



As I mentioned: not with Nikkor lenses, and as I mentioned, it was defined with yellow and blue; I was taught so about 35 years ago that CA was defined as yellow/blue artefacts.



Regards, Wim



P.S.: I am out of here, I don't have time for these discussions. As mentioned, I only wanted to add some hopefully useful info to this topic. You don't agree with me, but I didn't really expect anything else.



Cheerio.

[/quote]

Stop talking about desaturation, I have not mentioned desaturation.

You SHRINK the size of the offending channel. Lets say, red is projected slightly bigger. The red channel then needs to be shrunk, in order to match green and blue again. So.. while keeping the CENTER of the image in place, we shrink the image. Literally: The image may be 4000 pixels wide, and we might shrink the red channel to 3997 pixels, if that would be the amount needed to map red on the other information correctly again.



That is how one corrects LaCA. Some software does it that way, like Photoshop in its lens correction functionality, or DDP. Or you can do it by hand in PS too.



Some software or in-camera firmware does not correct CA, but masks it by desaturating coloured edges.



And yes, we do see the same thing, you just do not know what LaCA actually shows up as, so you label what you see wrong. Still it is the same thing we see, you and I.. PF.



Whoever taught you 35 years ago that CA was blue and yellow artifacts, taught you wrong. No biggie, as we are never too old to learn. For fun, I looked up "chromatische aberratie" in my "het objectiven boek" (the lens book) by Rudolf Smit, printed in MCMLXXIII, so 1973.

It mentions that blue gets bent ("gebroken") most in comparison to red, and from then on he just talks about red and blue edges around the contours of the image projected by the optics. Nothing about Nikon, nothing about yellow and blue artifacts being the definition of CA (LaCA), Nothing linking Nikon specifically to the definition of CA.

So, that was the state of art of explaining CA 35 years ago, and Rudolf Smit was teacher of photography at the "Amersfoortse Academie", did the test reports for the Dutch Foto magazin "Foto" and "Focus", if I am not mistaken.



On a side note, I have Nikon lenses from the 60's and 70's (ultra micro Nikkor 28mm f1.8, ultra micro Nikkor 55mm f2, original Nikkor 55mm f3.5 micro, Nikkor 35mm f2.8, Nikkor 50mm f2, Nikkor 85mm f1.8, Nikkor 135mm f3.5) and I have not noted a tendency to blue/yellow CA specifically, with them. Also looking at older designs being tested online, I tend to find red/green-blue CA (35mm f2, 24mm f2.8). 50mm f1.8 shows blue/yellow.



I know that whenever something you say gets corrected, you react this way. You do have the time for discussions, you just hate when someone disagrees with a part of what you wrote, or corrects part of what you typed up.

I do wish we could have normal discussions.



Anyway, sorry but what you labeled as LaCA simply is no LaCA at all. It shows no signs of being that in any way (lacks the directionality of LaCA, lacks the complementary colours, lacks the width and character of the coloured edges).

As such, it was not useful info.

[PuxaVida]

I tried to correct the image you loaded Ian...



CS4: Image Adjustments --> Hue / Saturation. Then select the channels and reduce lightness and saturation. The channels selected were magenta, green and blue respectively.



Oh, and I have to correct myself regarding the cause of PF. After a quick research on the net, there's no proof that the microlenses over the sensor cause PF. In general, people are divided in two:



- people say that PF is caused by the lens (because it is effected by chaning the focus in / out).

- people say that PF is caused by the sensor (because PF is less seen on film compared to digital).



And a few words about the recent discussion: the content was fruitfull and it could be even better if you BC could be careful about the way you contribute. I personally believe that you have a decent knowledge but with this way of approach they become less valuable than they deserve. I read posts of Wim for a long time here, and I never saw him refusing to step back concerning his arguements in case they are wrong (and I don't know if he is right / wrong in this case, this is not my point). Digital photography might be consisting of 0s and 1s, but social environments are not. I hope we can see posts from both of you here in future...



Serkan

[mst]

[quote name='Brightcolours' timestamp='1308181388' post='9271']

I do wish we could have normal discussions.

[/quote]



I certainly do wish that, too, BC, and most of all I wish you'd stop attacking people like you often do. For example in this post.



-- Markus

[mst]

[quote name='PuxaVida' timestamp='1308136602' post='9251']

Really? It would be good to know if you could give a bit more detail Markus...

[/quote]



I can't provide any detailed information or facts, just my plain opinion: how could a filter that blocks invisible light help to avoid such very visible flaws?



-- Markus

[mst]

[quote name='Brightcolours' timestamp='1308172389' post='9269']

The colour of CA needed further explanation too, as yellow/blue is quite rare.

[/quote]



Actually, I see yellow/blue CAs quite often in the test charts. And not only with Nikkor lenses.



In addition, CAs (laCAs) don't necessarily increase proportionally with distance from the image center. In fact, with many lenses they don't.



-- Markus

[Brightcolours]

[quote name='mst' timestamp='1308212606' post='9280']

I certainly do wish that, too, BC, and most of all I wish you'd stop attacking people like you often do. For example in this post.



-- Markus

[/quote]

Wim started the hostile wording, after I reacted to what he wrote about CA.

[Brightcolours]

[quote name='mst' timestamp='1308213070' post='9283']

Actually, I see yellow/blue CAs quite often in the test charts. And not only with Nikkor lenses.



In addition, CAs (laCAs) don't necessarily increase proportionally with distance from the image center. In fact, with many lenses they don't.



-- Markus

[/quote]

I said it is quite rare (after it was being said it most often is) and that you more often see red/greenblue or magenta/green... not that one does not encounter it?



CA's do increase proportionally with distance from the center, just not always totally proportionally. In the center one never will see it (unless the lens would have serious decentering issues I guess.

I did write that some don't have a nice and gradual CA shift, though, making the CA not so well correctable (but them one can mask it still to get rid of the coloured edges). We do not disagree here.