Lens terminology

[Jim Grimes]

What does APO stand for in a lens description?

[Steve Hamley]

Apochromatic

Steve

[steve simmons]

It means that a lens is designed to focus all three colors (RGB) at the same point behind the lens. A non APO lens is designed to focus two of them and then let depth of field take care of the third one.

steve simmons

[paulr]

these are approximate, idealized definitions. the actual definitions used by lens designers are very complex. they have to do with how close is close enough to perfection, for different kinds of chromatic aberration at different wavelengths.

in general, you can expect an apo lens to be somewhat better corrected for chromatic aberration than an equivalent non-apo lens. the differnces are more likely to be significant at wider apertures and at points far from the lens axis.

keep in mind that chromatic aberrations are just a couple of the many, many things that degrade an image.

[Eric_6227]

Is the difference really noticable to the eye? In color or sharpness? Does this have an effect in black and white?

[steve simmons]

In the old days with color the answer is probably yes. But in the last 15 years most lenses are so close that there is minimal, if any practical differefne in most outdoor situations.

steve simmons

[Emmanuel BIGLER]

Achromat, Apochromat, Superachromat - What is the Difference?

by Dr. Hannfried Zügge, group manager, Carl Zeiss optical design department

Carl Zeiss Camera Lens News, issue #12, winter 2000

www.zeiss.de/C12567A8003B58B9/Contents-Frame/2D4B2DCF22C996C3C12569BC005C33AF (http://www.zeiss.de/C12567A8003B58B9/Contents-Frame/2D4B2DCF22C996C3C12569BC005C33AF)

quote :
In most cases, the third zero is not required in practice, but it is sufficient to reduce the secondary spectrum to meet the respective requirements. If this is successful, such a lens could be termed "Apo lens".
end quote

To make a long story short, at least as I feel about this issue

At the beginning there was the Original Apochromatic definition (three
wavelengths brought to the same focus) by Enrnst Abbe for microscope
objectives.

Then the definition was extended to other kinds of lenses including 4-element symmetrical repro lenses of the dialyte type like the apo-artar and the apo-ronar. By construction, and when used at 1:1 ratio, those perfectly symmetrical lenses are free of lateral chromatic aberration (plus : coma-free and distorsion-free). So if, extra, the longitudinal chromatic aberration is "folded" three times, there are some good reasons for the image to be quite good. At least at 1:1 ratio, the miracle being that the image delivered by those venerable lenses is still damn' good even at infinity and not only at 1:1 ratio.

At a certain time at the end of the XX-st century, there was a German DIN Standard that authorized (German ?) manufacturers to label "APO" lenses that would meet certain requirements in terms of aberration corrections, but this was substantially different from the original definition edicted for microscopes one century ealier.

I would not be surprised if after that, again at the end of the XX-st century, previously non-apo lenses would become immediately eligible for the apo prefix... without any internal change.

The authoritative article by Dr. Hannfried Zügge clarifies, IMHO, the subject, i.e. we should not remember too precisely the original definition by Ernst Abbe as far as top-class photographic lenses are concerned.

Just use 'em and enjoy.
Whether the longitudinal chromatic aberration is actually folded twice or three times ;-);-)

[Emmanuel BIGLER]

From the vintage apo artar brochure :

www.schneiderkreuznach.com/archiv/pdf/ar_apo_90_240.pdf (http://www.schneiderkreuznach.com/archiv/pdf/ar_apo_90_240.pdf)

quote :

The lenses have been apochromatically corrected for the visible region from 400 nm to 700 nm as per DIN 19040, Page 5.

Now we are all set.

DIN 19040 standard page 5 is where the story comes from ;-)

Now who will pay to get a copy of it ? (usual and useless grumble : standards should be free !!)

[paulr]

"At the beginning there was the Original Apochromatic definition (three wavelengths brought to the same focus) by Enrnst Abbe for microscope objectives."

to this day, the technology to perfectly correct longitudinal chromatic aberration only exists for microscope optics. this is why the standard had to be changed for camera lenses. i believe they can meet the original standards for lateral chromatic aberration, but with longitudinal, they have accepted a compromised standard ... it's like a deviation from perfect focus by a certain percentage of the focal length, or by a certain number of wavelengths of light. not sure how the din standard works, but not too long ago different lens companies used different types of measurement.

lenses corrected to a certain standard over an even wider range of wavelengths are called superchromats.

"Is the difference really noticable to the eye? In color or sharpness? Does this have an effect in black and white?"

Probably, in some circumstances. Although it would be very hard in most cases to know if you're seeing the results of apochromatic correction specifically, or just the combined effects of many improved corrections in a newer lens design. Most modern apo lenses have a bit more coverage, and sustain better performance off axis than their pre-apo predecessors. The difference is usually slight, but off axis, especially at wider apertures and big enlargements, it can make a difference. Chromatic aberration needs to be pretty extreme for you to actually see color fringes in most real world situations (except with digital cameras, where color fringes can be exaggerated in some cases). Typically chromatic aberration would just be one of many factors that makes fine detail look softer. This is the case with both color film and black and white (most bw film is panchromatic).

[Jim Galli]

Do I remember reading somewhere that the Kodak Commercial Ektar's of the late '40's forward are all APO? And Goerz Dogmar's were in the 1930's?

[paulr]

one thing manufacturers don't make obvious is that an apo lens only meets the apo criteria within a pretty narrow range of magnifications. process lenses like apo tessars and g-clarons are apochromatic at or near 1:1 magnification, but not much beyond that. none of these lenses is at all apochromatic at infinity. just as an apo symmar is not apochromatic at 1:1.

[paulr]

good ol' wikipedia has some helpful entries.

notice the graphs (which represent longitudinal chromatic aberration). in a perfectly corrected microscope lens, the line representing the point of focus would stay right at zero. in other lenses, the focal point wanders, but stays within a certain tolerance. vague ideas like "correctted for 3 colors" are represented by the number of zero-point crossings. the achromat has two; the apochromat has three (and a line that stays closer to zero); the superchromat has four, (and stays closest to zero).

http://en.wikipedia.org/wiki/Superachromat

http://en.wikipedia.org/wiki/Apochromat

http://en.wikipedia.org/wiki/Achromat

[Bill_1856]

Jim, I think you were right abut the Dogmars. That's why they were used on 3 color seperation cameras of Curtis and National. They were also f:4.5, when every little bit of speed counted with the beam-splitters and filters.

[GPS]

"Is the difference really noticeable?" You can look at it this way - were it not, would there be any reason to go to such a length so as to add more elements to the lens and expense for nothing?? Just for the heck out of it?

[Turner Reich]

I have wondered what Artar is, as in Goerz Artar. Are there other Artar lenses and is it a design and not a name?

tr

[Ernest Purdum]

For many years, the abbreviation APO was found on "proces" lenses used for reproduction work. As Emmanuel Bigler mentioned, many of these were "Dialytes" which have four airspaced elements and are very tolerant of differences in subject/image ratios. Some others were Tessar types, highly asymmetrical and not at all tolerant of ratio variations from that intended.

Later, as space-saving vertical copy cameras came into common use, six-element symmetrical apo lenses were provided to provide the wider field these cameras need. The Schneider G-Claron is the most common of this type. Also general pupose lenses with the apo designation, the Apo-Lanthar, for example, started to appear.

[BrianShaw]

I have wondered what Artar is, as in Goerz Artar. Are there other Artar lenses and is it a design and not a name?

tr

Artar is a 1904 (ish) lens design of W Zschokke of Goertz. It is a symmetrical apo process lens comprised of a 4-element dialyte design (air-spaced achromatic design) which had positve elements made of barium glass (dense) and negative elements made of flint glass. It was the basic Goertz process lens for about 70 years.