What is chromic aberration? at DVinfo.net

Nathan Quattrini · April 18th, 2007, 08:53 AM

I was reading about it in camera comparisons and have no idea what it is or means. Thanks.

John Hudson · April 18th, 2007, 10:12 AM

It is Chromatic aberration

... In optics, chromatic aberration is caused by a lens having a different refractive index for different wavelengths of light (the dispersion of the lens).

http://en.wikipedia.org/wiki/Chromatic_aberration

The best example is looking through a glass of water as you drink it. You can see CA perfectly illustrated

Ervin Farkas · April 18th, 2007, 10:13 AM

"In optics, chromatic aberration is caused by a lens having a different refractive index for different wavelengths of light (the dispersion of the lens). The term "purple fringing" is commonly used in photography, although not all purple fringing can be attributed to chromatic aberration.

Longitudinal and lateral chromatic aberration of a lens is seen as "fringes" of color around the image, because each color in the optical spectrum cannot be focused at a single common point on the optical axis.

Since the focal length f of a lens is dependent on the refractive index n, different wavelengths of light will be focused on different positions. Chromatic aberration can be both longitudinal, in that different wavelengths are focused at a different distance from the lens; and transverse or lateral, in that different wavelengths are focused at different positions in the focal plane (because the magnification of the lens also varies with wavelength)." - From Wikipedia.

A. J. deLange · April 23rd, 2007, 06:32 AM

The fact that light of different colors are refracted to different extents (the reason why a prism produces a spectrum) means that the focal length of an uncorrected lens will be different for each wavelength of light. This has 2 efffects. For a given lens setting, light at the more refracted wavelength will focus at a point closer to the lens than light at a less refracted wavelength. This means that while the portion of the image caused by one wavelength may be in focus portions from another wavelenth will not be. This is longitundinal chromatic aberration and results in loss of sharpness in the image (the MTF of the lens is impaired). Second, the size of an image is proportional to the focal length of the lens which forms it. Thus, in a lens with CA, image components have a size which depends on the color of the light which forms them. This results in fringing which appears radially i.e. a blue fringe may be found on the outer edges (away from the center) of objects while inner edges (towards the center) may show a red fringe (the actual colors depend on the particular lens and the wavelengts at which it is under or over corrected). The degree of fringing gets worse the farther from the center of the image it is located. This is lateral chromatic aberration.

Lateral CA can be corrected in post processing to some extent by simply adjusting the relative sizes of the red, green and blue images. This is done frequently with programs like Photoshop and serves as an excellent test of whether fringing is caused by CA or another cause. People tend to blame any fringe on CA but that is not always the case. If you can't improve it by shrinking or expanding the three color channels realtive to one another, it's not CA. A simpler test is observing whether the fringe is to the inside - outside as opposed to left side or right side. If it isn't radial, it isn't CA.

Longitudinal CA, conversely, cannot be corrected without extremely elaborate software.

Nathan Quattrini · April 23rd, 2007, 01:10 PM

so the beginning of this video along the windows on the left and the green outline on the first house on the right...would that be it? And is there a way through settings etc to not have white do that so much?

http://www.dvinfo.net/conf/showthread.php?t=92132

A. J. deLange · April 23rd, 2007, 05:09 PM

I don't see a green outline on the right but I do see a vertical green bar that runs from the top to the bottom of the picture. Is that what you are referring to? If so that is definitely not CA because it is not radial (it would be worse at the top and bottom were it CA) and there is no matching bar at the left edge of the picture. Futhermore, the red, green and blue images all appear to be the same size to me

These cameras have CA at about the pixel level so it is unlikely that we'd be able to see it in a reduced resolution version of the video.

Nathan Quattrini · April 24th, 2007, 08:11 AM

if you look at the far edge of the first house on the right it has a turquoisey glow to it. And the first house on the lefts window frames have a reddish purple 'fringe' i guess its called.

April 18th, 2007, 08:53 AM	#1
Nathan Quattrini Major Player Join Date: Jan 2007 Location: Warwick, Rhode Island Posts: 740	What is chromic aberration? I was reading about it in camera comparisons and have no idea what it is or means. Thanks. __________________ Cinematography Site

April 18th, 2007, 10:13 AM	#3
Ervin Farkas Inner Circle Join Date: Aug 2005 Location: Atlanta/USA Posts: 2,515	Did you mean chromatic aberration? "In optics, chromatic aberration is caused by a lens having a different refractive index for different wavelengths of light (the dispersion of the lens). The term "purple fringing" is commonly used in photography, although not all purple fringing can be attributed to chromatic aberration. Longitudinal and lateral chromatic aberration of a lens is seen as "fringes" of color around the image, because each color in the optical spectrum cannot be focused at a single common point on the optical axis. Since the focal length f of a lens is dependent on the refractive index n, different wavelengths of light will be focused on different positions. Chromatic aberration can be both longitudinal, in that different wavelengths are focused at a different distance from the lens; and transverse or lateral, in that different wavelengths are focused at different positions in the focal plane (because the magnification of the lens also varies with wavelength)." - From Wikipedia. __________________ Ervin Farkas www.AtlantaLegalVideo.com

April 23rd, 2007, 01:10 PM	#5
Nathan Quattrini Major Player Join Date: Jan 2007 Location: Warwick, Rhode Island Posts: 740	so the beginning of this video along the windows on the left and the green outline on the first house on the right...would that be it? And is there a way through settings etc to not have white do that so much? http://www.dvinfo.net/conf/showthread.php?t=92132 __________________ Cinematography Site

April 24th, 2007, 08:11 AM	#7
Nathan Quattrini Major Player Join Date: Jan 2007 Location: Warwick, Rhode Island Posts: 740	if you look at the far edge of the first house on the right it has a turquoisey glow to it. And the first house on the lefts window frames have a reddish purple 'fringe' i guess its called. __________________ Cinematography Site

April 18th, 2007, 10:12 AM	#2
John Hudson Major Player Join Date: Sep 2003 Location: 32° 44' N 117° 10' W Posts: 820	It is Chromatic aberration ... In optics, chromatic aberration is caused by a lens having a different refractive index for different wavelengths of light (the dispersion of the lens). http://en.wikipedia.org/wiki/Chromatic_aberration The best example is looking through a glass of water as you drink it. You can see CA perfectly illustrated

April 23rd, 2007, 06:32 AM	#4
A. J. deLange Major Player Join Date: Jun 2004 Location: McLean, VA United States Posts: 749	The fact that light of different colors are refracted to different extents (the reason why a prism produces a spectrum) means that the focal length of an uncorrected lens will be different for each wavelength of light. This has 2 efffects. For a given lens setting, light at the more refracted wavelength will focus at a point closer to the lens than light at a less refracted wavelength. This means that while the portion of the image caused by one wavelength may be in focus portions from another wavelenth will not be. This is longitundinal chromatic aberration and results in loss of sharpness in the image (the MTF of the lens is impaired). Second, the size of an image is proportional to the focal length of the lens which forms it. Thus, in a lens with CA, image components have a size which depends on the color of the light which forms them. This results in fringing which appears radially i.e. a blue fringe may be found on the outer edges (away from the center) of objects while inner edges (towards the center) may show a red fringe (the actual colors depend on the particular lens and the wavelengts at which it is under or over corrected). The degree of fringing gets worse the farther from the center of the image it is located. This is lateral chromatic aberration. Lateral CA can be corrected in post processing to some extent by simply adjusting the relative sizes of the red, green and blue images. This is done frequently with programs like Photoshop and serves as an excellent test of whether fringing is caused by CA or another cause. People tend to blame any fringe on CA but that is not always the case. If you can't improve it by shrinking or expanding the three color channels realtive to one another, it's not CA. A simpler test is observing whether the fringe is to the inside - outside as opposed to left side or right side. If it isn't radial, it isn't CA. Longitudinal CA, conversely, cannot be corrected without extremely elaborate software.

April 23rd, 2007, 05:09 PM	#6
A. J. deLange Major Player Join Date: Jun 2004 Location: McLean, VA United States Posts: 749	I don't see a green outline on the right but I do see a vertical green bar that runs from the top to the bottom of the picture. Is that what you are referring to? If so that is definitely not CA because it is not radial (it would be worse at the top and bottom were it CA) and there is no matching bar at the left edge of the picture. Futhermore, the red, green and blue images all appear to be the same size to me These cameras have CA at about the pixel level so it is unlikely that we'd be able to see it in a reduced resolution version of the video.