I think a better way to explain it is that lenses with CA have different focal lengths for different wavelengths. The net result is that the red green and blue images recorded on digital are not the same magnification even if the depth of field at the shooting aperture allows all three colors to be in focus on the sensor. By slightly changing the magnification of two of the colors to match the third, the CA can often be eliminated in post processing. CA is most noticeable off axis because of the fast that the magnification differences are most noticeable the further away you look from the center of the image. jco
-----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Godfrey DiGiorgi Sent: Tuesday, November 14, 2006 11:52 AM To: Pentax-Discuss Mail List Subject: Re: Chromatic Aberration On Nov 14, 2006, at 7:59 AM, Shel Belinkoff wrote: > It was recently mentioned that some pics that were posted here and > which > were made with a K18/3.5 had CA. I looked and couldn't see any, and > haven't seen any with my 18/3.5. Maybe I don't know what to look for. > > Can someone explain (in simple terms) what CA is, how it can be > recognized, > and perhaps provide a URL to an example or two. Thanks. Chromatic Aberration is what happens when different wavelengths of light passing through a lens do not come to focus at the same point, particularly off-axis. The most typical case of CA is red-green shifting ... if you look closely at well defined corners/edges of a typical photo, with a lens exhibiting CA, you'll see a red fringe on one side and a green fringe on the other. Yellow-blue shifting is also observable in many lenses but is more difficult to see as it is often masked by red-green shifts. If you look carefully at the pipe at the upper left corner of this DA14mm exposure <http://homepage.mac.com/godders/14mm-examples/sRGB-8Bit/ IMGP2144-8bit-sRGB.jpg> you can see a little bit of red-green shifting. Red-green and blue-yellow CA is easily reduced and/or eliminated in Adobe Camera Raw and other RAW converters, using the Lens tab controls. (I don't think the Lens tab is available in Photoshop Elements, but I'm not certain.) A little bit of residual chromatic aberration is there in nearly any lens. Most of the time it is not very obvious at print resolutions. BTW: An "apochromatic" (APO) lens is a lens which has been certified that the three primary wavelengths (Red, Green, Blue) focus to the imaging plane simultaneously. CA is different from sensor blooming (also referred to as red or purple fringing). Blooming is an effect caused by interactions at high contrast boundaries on a digital sensor, caused by electrical crossover effects at the photosite wells in conjunction with interactions of the anti-alias filter and bayer demosaicing. CA can exacerbate blooming but is not the root cause. It is usually a pink- magenta-purple color and does not shift side-to-side like CA. Blooming is more difficult to remove and is often indicative of a slight mismatch between the optical system and the sensor. Some lenses push the sensor to produce more blooming... Godfrey -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net
