CA - contrast - sharpness?

kansalliskala · Expire: 2016-12-30

How they relate to each other?

Does anyone have some not-so-advanced-in-physics -type practical reading links about this?

no-X · Posted: Tue Dec 01, 2009 9:38 am Post subject:

As I remember, Orio posted some nice links earlier. There was also a nice article by Zeiss about relation of subjective sharpness vs. resolution vs. contrast.

Anyway, CA isn't directly related to sharpness or contrast, but there can be indirect influence. CA can be sometimes "masked" by low contrast/saturation of old lenses or by other optical aberration, which blurs final image (together with signs of CA). Also simple diffraction (which blurs image at high f-stops) can blur-away some CA, too Smile

cooltouch · Joined: 15 Jan 2009 Posts: 9096 Location: Houston, Texas

When it comes to CA and sharpness and contrast, here's what I keep in mind:

In general terms, chromatic aberration occurs with a lens when the different wavelengths of light do not have the same point of focus. Red's wavelength is longer than blue, for example, so the challenge for optical designers is to get wavelengths of varying values to meet at the same point of focus. When they don't, we see color fringing. But also when they are not meeting at the same point of focus, this necessarily means that the lens is losing both sharpness and contrast. It's losing sharpness because components of the image are not at the same focal plane as others. It's losing contrast for much the same reason.

This is one reason why lenses that use low-dispersion glass or aspherical elements or apochromatic designs are often sharper than equivalents that don't. This is also why lenses of smaller maximum apertures often provide sharper images than those of larger maximum apertures, because it's easier to design a slower optic with fewer CAs that a faster one.

spiralcity · Joined: 02 Oct 2008 Posts: 1207 Location: Chicago, U.S.A

Heres a couple of links with some info on CA.

http://www.dpreview.com/learn/?/Glossary/Optical/chromatic_aberration_01.htm

http://hyperphysics.phy-astr.gsu.edu/HBASE/geoopt/aber2.html

This is a link about lens contrast.

http://www.luminous-landscape.com/tutorials/understanding-series/lens-contrast.shtml

This is Ken Rockwells take on sharpness.

http://www.kenrockwell.com/tech/lens-sharpness.htm

Heres a link on doing a few test.

http://www.bobatkins.com/photography/technical/lens_sharpness.html

francotirador · Joined: 17 Sep 2009 Posts: 894

Very good information, thanks Laughing