What Does the Word 'APO' on a Lens Really Mean?

Written by Henning Wulff

The term 'apochromat' gets used in various ways, and now it means various things.

Years ago, it just meant that a lens focused three different colors on the same plane and at the same magnification at a given focusing distance. Reproduction houses depended on such lenses to produce separations for color, and therefore the three colors had to be the three that were used in their process. It mattered not what happened to the other wavelengths, or at other focusing distances. Therefore old 'APO' process lenses are not necessarily decent general purpose lenses. I know; I have had some.

Basic chromatic aberration is the failure of a lens to focus all colors to the same convergence point. Another way of saying the same thing is that identical objects of different colors would be imaged at the film/sensor plane at slightly different sizes.

Then the average consumer got dragged into this by the marketing departments. APO came to mean not just identical magnifications and focusing planes for three colors, but excellent chromatic correction overall and general excellence. So then, often, apochromatic referred to the overall imaging excellence, with correction that was excellent but not necessarily apochromatic in the old sense. For general use at various focusing distances, these new APO lenses were of course a lot better than both the old APO lenses and the same manufacturers' non-APO lenses. Nowadays, when a manufacturer like Cosina labels a lens 'APO,' it had better be excellent and have outstanding longitudinal chromatic aberration (LoCA) and total chromatic aberration (TCA) correction, as the latter is very visible on digital sensors when not corrected.

There is also the superachromat type of lens, which focuses four wavelengths on one plane, and hopefully also keeps the magnification of those four wavelengths identical. Then we have a truly outstanding lens with respect to color correction. Designers, specifically the Zeiss designers that worked on the superachromat, specify that not only those criteria have to be met, but that tolerances and sample deviations have to be addressed to such a degree that the resulting lenses have to be truly outstanding, for the lens to be labeled superachromat. Because of the wide wavelength correction, these lenses tend to be excellent into the IR. The Coastal Optics 60mm [AKA Jenoptik UV-VIS-IR 60mm ƒ/4, about $5,750 —Ed.] would fall into this category, even though it is not specifically labeled as such.

So now the purely technical definition of apochromat isn't sufficient; it has to be outstanding overall, not just with respect to the specifics of the original definition.

Now, with marketing hype, consumer expectations, and historical antecedents, APO has come to mean a number of things, and manufacturers respond to them in various ways and have various standards for their application. The Zeiss Otus lenses are probably mostly APOs for most manufacturers, but Zeiss doesn't directly label them as such. Some of Leica's lenses could be termed APOs but aren't. Leica lenses that are labeled APO definitely are. Early Sigma APO lenses were only APO in the most limited sense. I only have one Cosina/Voigtländer lens labeled APO, and that's a 180mm ƒ/4. It's a bit shaky as an APO. On the other hand, what I have seen from some of their other lenses would seem to indicate that some are reasonably labeled 'APO.'

Henning

[Henning submitted this as a comment to John's recent "Nicest Lens I've Ever Owned" post, but not until a number of days had passed. Since few people see new comments on older posts, I asked him if I could publish this separately so more people would see it. In accordance with house style I have reverted to American spellings and typesetting; I also added the illustration and its caption. Thanks to Henning! —Ed.]

©2017 by Henning Wulff, all rights reserved

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Henning Wulff adds: "In a couple of ways film is less sensitive to color aberrations than digital sensors are.

"All lenses, whether monochromat, achromat, apochromat or superachromat, have color aberrations, just as they all have all of the other aberrations. You might have to look harder or use them under more stressful conditions to find them, but they all have every aberration. General purpose lenses have a tough time.

"With high resolution digital sensors most of these become more easily visible, especially with 200% magnification. We see things that really shouldn't be seen photographically, only in a technical, non-photographically super nit-picky way. Make a print and see if you like it, or massage it so it looks good in a blog. If the results still look bad, it's probably due to more things than chromatic aberrations.

"When we shot color negative film, which by its very nature is not high acutance and a bit lumpy, lenses that would have created colour fringing due to either longitudinal color aberration or color aberration due to different colors being magnified at different ratios, these were gradual effects that were swallowed up in the general mush. There were of course higher acutance films, such as the slower Kodachromes and Ektachromes that exposed these fault at least to the degree that images shot with sub-optimal lenses weren't as 'crisp.' With digital images the sensors next to the ones that should be the ones defining the edge also get some of the color aberrations and then we get 'fringing' on top of the general softness of less than fully corrected lenses.

"Remember—all lenses have all aberrations unless used at specified wavelengths (we're about maxed out at six right now) at specified magnifications. Actually, those lenses still have most aberrations, except not the main color ones.

"The main thing to take away from this is that now 'APO' means better lenses, but 'APO' also means different things to different marketing departments. Buyer beware."

Comments

[toto]

Would color film and Foveon sensors be better served by microscopically non-APO lenses, being that the light-sensitive layers are at microscopically different depths?

[David Brown]

How did we ever manage to take color photographs (on film) for decades without APO lenses? Serious question!

[Tom Kwas]

I can tell you that Goerz Red-Dot Artars were certainly sharp for product photographers, I would say sharper than almost any other lens. They were sharper than my Schneiders, and slightly sharper than my Nikkors, at least for product photography, at product photography distances. They were "APO", hand assembled, but also "set" for sharpest focus not at 1:1, but at various distances depending on the focal length. I once looked at the specs for the version of the 8.25" I had, it was dead-perfect for the magnification of shooting a coffee maker onto 4X5!

Most view camera lenses were set for infinity focus, hence part of the Red-Dots sharpness was based on the fact that they were set for closer distances. I get the impression, that the "APO"-ness of the lenses were also based on their prime magnification setting, i.e., maybe an "APO" Red-Dot, wouldn't be quite so "APO" at infinity, as it was at 1:5 / 5:1!

Digital be damned, I still have decent Red-Dots in 8.25, 10.75, and 12, all in Compur shutters!