What does CA stand for?

I missed the initial use of the term "CA" in an earlier post. What does it stand

for?

 

Jim

Chromatic Aberration

Chromatic Aberation. Most often seen as a bit of purplish or yellowish fringing around high contrast items (like tree branches against a bright sky), and often in wider-angle lenses, all the more so at the edges of the frame. Modern image editing tools can take care of it pretty easily.

And "California" :) Sorry!

Well, California in the context of United States state names or country code for Canada. But in the context of photography ....

Jim, any lens slightly changes the path of colors in light (due to the very nature of

the medium at play, refraction indexes, etc.). Due to this, different wavelengths

(colors) don't arrive exactly at the focal plane (fim or sensor). Some colors focus

ahead of the focal plane and other focus behind it, resulting in chromatic aberration,

as it is named. Usually, a combination of negative and positive lenses (achromatic

optic system) minimizes the aberration, albeit not completely. Standard lenses are

achromatic.

</p>

Only by means of using extra dispersion glass or fluorite can you obtain an APO

(apochromatic) lens that is virtually free from CA, and I say virtually becase some

residual CA can even be present in an APO system, although usually not detectable

for human eyes.

</p>

Having said this, you can always diminish the CA in a lens by means of stopping

the lens. Also, long focal lenses suffer more from CA that wideangle lenses. Finally,

I would like to seize the opportunity for saying that against popular belief, aspheric

lenses and CA have nothing in common. Aspheric lenses correct other optical

faults, but not CA. Finally, for obtaining the qualification, any APO lens is also

corrected against spherical aberration and astigmatism.

Oh, I forgot: Fluorite and ED glass is expensive, so eliminating CA will never be cheap.

In Photoshop CS3, the path Filter>Distort>Lens Correction has a software fix for CA, as well as for various other lens related problems.<div></div>

Could be Canned Air, Cash Advance, Captain America, Citric Acid, or Caramel Apple. See if they fit!

CAnon + CaF2(Calcium Fluoride) defeats CA(Chromatic Aberration).

Matt - (warning ... thread hijack here by me) ...with reference to your statement, is chromatic abberation more visible in wide angles? The only time I've really noticed it is in the tele shots of tree branches,etc. Is it just that it's easier to see in a twig vs the sky, or do the teles have this problem more than wides? Thanks. Jim

"CA" is a real issue especially with tele lenses 300MM or longer. Certain wavelengths of light (particularly reds) will actually focus further from the film plane than the other colors. In the 70's there were many aftermarket junker lenses that were wicked with this affliction.

 

I once saw a piece of 16MM movie film shot with a lens that was incapable of focusing red due to severe CA! A woman's red dress was constantly soft as she moved about the frame.

Chromatic aberration is not just one phenomenon. It encompasses the whole range of aberrations affected by variation in wavelength. Often they are stated as either lateral or longitudinal chromatic aberration. Longitudinal, or ability to reach a common focus, can be fairly well corrected by proper choice of glasses. Lateral, or variation of image magnification with wavelength, is usually more difficult to correct and often requires special low dispersion or anomalous dispersion glass or crystalline calcium fluoride (fluorite) lenses. These are both expensive and fragile. This is one reason those chunks of "Big Glass" are so dear.

Well, Jim, I think we can do some research here... but many discussions here do revolve around the CA seen (or not!) on various ultra-wides. I can tell you that when I shoot out at longer focal lengths (say, 300+mm) with a quality lens, I don't find myself fussing about CA the way I do with a rectilinear ultrawide (say, at 10mm). One of the great glowing wonderfullnesses reported by users of the new Nikon 14-24 is the lack of CA for a lens that shoots that wide on a full-frame body.

 

I'm sure some others will chime in, here. Steve's right that any time you really start bending light, different frequencies are going to give you fits. Very wide angle lenses do some real bending, just in a different configuration than very long lenses. Same issue, different manifestation. In practice, it's only been something I've had to think about (and clean up after) with wider lenses.

The country code for Canada was supposed to be just a "C" but when the Canadians saw it they said: "C, eh". So it stuck

Thanks Matt and others; every time I stop in here I learn something and have my brain stretched a tad. You guys are so darned refreshingly bright and friendly, it's a joy to read the diverse comments. Thanks P-Net, et al. Jim M.

If your from Boston, MA it is what you drive, everyone I know drove a CA.

CA - Heavy Cruiser!