06-15-2011, 08:57 PM
[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.
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.
Selective desaturation IOW.
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
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.
Quote: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.
Quote: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.
Quote: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
Gear: Canon EOS R with 3 primes and 2 zooms, 4 EF-R adapters, Canon EOS 5 (analog), 9 Canon EF primes, a lone Canon EF zoom, 2 extenders, 2 converters, tubes; Olympus OM-D 1 Mk II & Pen F with 12 primes, 6 zooms, and 3 Metabones EF-MFT adapters ....