What is an APO lens??

[Tom O]

OK, am I the only one here that doesn't know what APO stands for??

Could it be:
Aperture Priority Operation
Aspherical Programmed Option
Auto Perspective Orientation
Aimless Promotion Opening
Absolute Perfection Only
Aiming Persistence Outlining
Asymmetrical Peerless Operation
Ashamedly Par Output

I've searched on it but found nothing.

Someone clue me in...

--
Tom O
http://www.pbase.com/tom_and_kris (but most pics w/Canon G1)

 

[Adria]

It's not an acronym, it's an abbreviation, and it's Sigma's equivalent of Nikon's "ED" specification.

From Sigma's website:

APO lenses. These lenses use apochromatic design and Special Low Dispersion glass (SLD) for minimum color aberration and ultimate telephoto image quality, improving contrast and sharpness.

Apochromatic (APO) telephoto lenses greatly minimize color aberration, a phenomenon created by different wave lengths of light refracting at different angles. This causes each color to have a slightly different imaging point, resulting in poor image quality.

--Adria
--
http://www.pbase.com/adriainpdx

 

[RichardCurDeLion]

Can't explain specifically what the APO stands for but this is copied from Sigma's web site under APO. Hope it helps

In order to attain the highest quality images, the APO lens has been made using special low-dispersion (SLD) glass and is designed to minimize color aberration.

SIGMA's APO zoom lenses minimize color aberration. As the refractive index of glass depends on the wavelength of light, color aberration occurs when different colors form images at different points. This problem often occurs with telephoto lenses, but the Special Low-Dispersion (SLD) glass and Extraordinary Low Dispersion (ELD) used in SIGMA's APO lenses helps to compensate for color aberration, thereby allowing them to produce of sharp images.

Richard Cur-de-Lion

'My lance is always ready'.
I'm often away crusading - so try me on my mobile, which is Whitehall 1212

 

[Tom O]

In my defense, I guess I thought it was an acronym since all 3 letters were capitalized.

--
Tom O
http://www.pbase.com/tom_and_kris (but most pics w/Canon G1)

 

[Tom O]

--
Tom O
http://www.pbase.com/tom_and_kris (but most pics w/Canon G1)

 

[John4]

OK, am I the only one here that doesn't know what APO stands for??

At the risk of telling you some information you probably already know:

Chromatic abberation is where a lens breaks up the light into a spectrum of colors like a prisim.

'Achromat' (literally 'without color') is a lens where two of the colors, typically red and blue, are brought back together to the same focus by using different types of glass in the lens. With this type of correction, you can often still see some residual abberation, e.g., green on one side and purple (deep red plus deep blue & violet on the other edge).

'Apochromat' is a lens where three colors are brought back together. There is still residual color aberration of the colors other than the three corrected for but it is usually less than with an achromat.

The situation is complicated nowadays because of the special ED and similar glasses available which permit achromatic (two colors) correction that is as good as apochromatic (3 colors) used to be. hence some manufacturers are using the term loosely when two colors are corrected but the correction is very good.

Why aren't all lenses apochromatic? Because when you use up your design freedom correcting for three colors, other aberrations cannot be corrected for as well. Lens desigh is a big pot of compromises.

Whew. I hope all this typeing helps.

Regards, John

 

[Tom O]

VGE = Very Good Explanation!!

And fear not, you were not telling me something I already knew.

Thanks again!
--
Tom O
http://www.pbase.com/tom_and_kris (but most pics w/Canon G1)