Apo Sironar not apochromatic?

[Daniel Geiger]

I just ordered a Rodenstock APO sironar S 210 mm to go between the 150 mm sironar and the 360 Nikkor T. [Buyer's remorse: should I have gone with a 240 instead? Please don't tell me ;-)]. Anyway, looked at the Rodenstock tech data, and the chromatic correction chart shows the curve passing zero twice, around 438 nm (blue) and around 540 nm (green). I thought the term "apochromatic" means corrected for THREE wavelengths, while corrected for two wavelengths is "achromatic". Am I missing something? Is this some sort of terminology inflation in LF-gear? My microscopy lenses (Zeiss planapos) are truly apochromatic.

The lens seems to be quite fine according to the messages and reviews I've seen, so I'm not worried about performance. Just wonder about some terminology.

Thanks in advance for the collective wisdom.

[Bob Salomon]

For camera lenses the definition for Apo is that the lateral chromatic aberrations of the secondary spectrum have been reduced/eliminted to within a minute percentage of the focal length. This eliminates color fringing, especially of finee details against a high contrast fisld.

The definition of apochromatic for micoroscope lenses is different and is the one where all three lines pass at a common point.

[Bob Salomon]

For camera lenses the definition for Apo is that the lateral chromatic aberrations of the secondary spectrum have been reduced/eliminted to within a minute percentage of the focal length. This eliminates color fringing, especially of fine details against a high contrast fisld.

The definition of apochromatic for micoroscope lenses is different and is the one where all three lines pass at a common point.

[Michael S. Briggs]

Yes, the two German LF lens manufacturers have changed the definition of the term "apo" for camera lenses.
They are referencing a German DIN standard for the term to mean tightly controlled chromatic aberation instead of the traditional definition of bringing three wavelengths to exactly the same focus.





This question has been asked many times in several forums and usually Bob Salomon responds and certain people rather agressively counter respond. Let's not get into those fights again. No one from Schneider ever says anything. At this point neither company could stop using the term because they would loose too many sales -- too many buyers would buy the competitor's product based on the name containing "apo".





Anyway, what is more important than the label is the performance of the lens. For the graphic arts trades the performance for the three primary colors was critical. For photography we care about all colors (including B+W photography unless we are using a strong filter or ortho film), so the new definition is more relevant than the old one.





The Apo-Sironar-S series should have better chromatic abberation since these lenses use ED glass with extra-low dispersion with anomalous partial-dispersion. Rodenstock provides graphs of the Longitudinal Color Aberration in their paper datasheets (a few are on the web in PDF form at http://www.linos.de/en/prod/obj_analoge_fotogr.html).
These graphs show the range of Longitudinal Color Aberration of the 210 mm Apo-Sironar-S to be about x1.7 less (i.e., better) than that of the 210 mm Apo-Sironar-N. It's interesting that Rodenstock publishes the data on the Longitudininal Color Aberation when Bob refers to the Lateral. As a practical matter, improving one probably tends to improve the other.



If you want to feel even better about your Apo-Sironar-S, read the comments of Chris Jordan, who is a very demanding photographer: http://www.largeformatphotography.info/lfforum/topic/501943.html.

[Daniel Geiger]

Dear Bob and Michael,

Thanks for the most illuminating information. Much appreciated!

Best wishes

[paulr]

"No one from Schneider ever says anything. At this point neither company could stop using the term because they would loose too many sales -- too many buyers would buy the competitor's product based on the name containing "apo"."

A Schneider technician I used to know said the same thing as Bob. He also said the difference is because it's possible to reduce longitudinal (is that the word?) chromatic aberation to zero in a microscope lens but not in a camera lens. This is probably a simplification of complex topic, but at any rate, I don't think anyone's being dishonest.

[Richard Schlesinger]

Question: what about "apo" as applied to enlarging lenses?

[Dan Fromm]

Richard, go take a look at Rodenstock's data sheets for Apo Rodagons.