Chromatic Aberration and other distortions

[S54toIS350]

I would like to humbly request a sticky thread on the types of distortion involved with photography and what causes it and how to get rid of it

So let me start by asking the question - Chromatic aberrations, what is it? and what causes it? Does a lens hood promote it?

[whoster]

lens hood doesn't promote CA

chromatic aberration happens when the light bending around a certain subject begins to refract--and the lens catches it as a purple fringe in the image. it usually happens when a subject is backlit by a strong source of light, or in general when it's brightly lit.

well-built lenses like your 24-70mm which has UD glass (a dispersion glass designed to eliminate CA) shouldn't, and doesn't really have a problem with CA, especially when stopped down to 5.6 or so.

lots of people mistake strong backlighting "halos" (which are blue, much to the color of the sky) as CA. CA is pretty much exclusively purple (unless Percy in his ginormous library shows us differently...).



as for the distortion.

there is barrel distortion, which rounds out the lines within an image outward.

and then there is pincushion, which is the curving inward of lines in an image.


zoom lenses by design have distortion--and it's not something you can get rid of unless you cover your entire focal length with primes. you CAN, however, get rid of said distortion easily in photoshop using the Lens/Glass Correction filter. it's just a matter of sliding a bar either left, or right.

[Percy]

CA exists in ALL lenses to a certain degree. Depending if it's slightly near or far from the focal point, you'll get different colors. Purple or Yellow. Even the best made refractive optics in the world have an extremely small amount of CA. Just way too many variables to completely get rid of it.

Now you could go to a mirror based system like Nikon did for their ultra telephotos way back in the 70s. Now the main thing that was problematic was the corrector plate up front (it caused CA!) and the mirror in the back often caused scatter problems. Remember that a aluminum coating or silver coating for a reflective surface will have scatter problems due to the fact that you're depositing small chunks of metal...it's never perfectly smooth. Thus you'll get a "hazy" effect and a contrast loss. With refractive optics (lens based instead of mirror based), you'll have other issues, but refractive optics provide the most contrast.

[hapaboy]

there's also spherical aberration.

to eliminate this, let's say, for example canon has developed 4 types of aspherical lenses.

ground aspherical lens
replica aspherical lens
glass molded aspherical lens
plastic molded aspherical lens

thus, reason to purchase nikon high end ED lenses (eventhough they've put ED elements in their cheapest lenses) and canon L series lenses.

[S54toIS350]

Originally Posted by hapaboy

there's also spherical aberration.

to eliminate this, let's say, for example canon has developed 4 types of aspherical lenses.

ground aspherical lens
replica aspherical lens
glass molded aspherical lens
plastic molded aspherical lens

thus, reason to purchase nikon high end ED lenses (eventhough they've put ED elements in their cheapest lenses) and canon L series lenses.

How much affect do camera software have on CA you know? I heard the digital backs have close to Zero CA.

[Percy]

The digital backs themselves won't introduce any Chromatic Aberration. It's the lenses. Remember that all lenses have a certain amount of CA, some more than others. From a software standpoint, it may be possible to get rid of most of the CA, but not all of it.

Getting rid of CA via software will bring up other problems. Viewing the entire frame of before and after will usually reveal problems in resolution. It's good for a "quick fix", but not the "end all". This is if you're being critical.

For astrophotography, having a lens that very nearly focuses perfectly in all the wavelengths is desirable to the ones that only partially focus in reference to the wavelength in question. There are wierd geometric errors that can occur during CA correction if the software isn't up to snuff. Many refractor type telescopes are corrected for 550nm, or the green area where the eye is most sensitive. The other colors are usually "acceptable" in correction. Commercially made optics such as the ones from Zeiss, Canon, Nikon, etc are made with a fairly large "fudge factor".

About the only lens that doesn't introduce any color error (refractor wise, not mirror based) would be the ones from Takahashi. Specifically the TOA and FSQ series. You can meter out the TOA on the test bench and it's close to color perfect as one can get, within 0.1mm of focus plane. Even the famed Astro Physics refractors are only accurate to within 0.3mm. Commercial optics have MUCH more slop than either one of the two mentioned optics.

http://www.geocities.com/grandeyehk/tsa130_fct250.htm

BTW, you can still get CA in a mirror based optic! The corrector plate up front (let's use the 500/8 Sony/Minolta for example) introduces the color error. Same with all of the Nikon SCT based ultra telephotos as well as many telescopes with a glass plate up front. If it didn't have the glass plate, or if it was very very well made, then you would have extremely little CA.

[Photoinc]

The Color Replacement tool is the perfect tool for removing the purple fringe or CA.
It's behind the Healing Brush in the tool pallet, in PS CS, CS2, and CS3

David