Are Nikkor-W lenses apochromatic?

[John Dorio]

From the looks of their design, which is quite similiar to the APO Symmar lenses one would think that the Nikkor-W lenses are also apochromatic. Are they? How s imiliar are the MTF curves for these two lenses? Would anyone care to comment?

[Michael S. Briggs]

The only design information I have seen for these lenses are their cross-sections. All that the cross-sections tell you is that these are plasmat or plamat-based designs. The plasmat design goes back to the early 20th century. As an example, the previous Symmar-S was a plasmat and wasn't termed apochromatic by Schneider. Whether apochromatic correction is achieved also depends on the glasses used. Without optical test equipment, the only way to discover this is from the patents, if such exist.

The only way to answer your question would be to decide ones own definition of apochromatic and then do optical bench testing. A better plan would be not to care about the word "apochromatic" and do photographic testing and decide whether the results satisfy you. I doubt that you will find the Nikkor-W unsatisfactory. Whether there is some small quality difference between the Nikkor-W and the Apo-Symmar, I don't know. One would have to test both lenses. At this level of quality, one must also consider the possibility of quality variations from lens to lens of the same brand.

The German manufacturers publish MTF curves. I haven't seen MTF curves for Japanese large-format lenses.

[Pete Andrews]

The only brochure that I could find for Nikkor large format lenses makes no claim of apochromatic correction for the Nikkor-W. I can tell you from using one for the last 15 years, however, that it's an excellent lens that'll stand up against anything else on the market. Apochromatic correction is really only an issue with long lenses for smaller formats and lenses used at the extremes of their image circle. Even then you'd have to be doing some serious enlargement before you saw the difference between an Apo and normal Achromatic lens.

[Pat Raymore]

If I understand the definition of an Apochromat correctly, it is a lens that brings three difference wavelengths of light into commom focus. A superchromat brings four difference wavelengths into common focus. Now the actual details are a bit more complex, but by this definition is the Nikkor W an apochormat? No. Is the apo symmar or the Sironar S an apochormat? NO. They have a reduced secondary spectra but are not apochromatic. Is the Apo Rodagon or Apo componon an apochromat? NO. But they do have a reduced secondary spectra compared to their siblings. Is the apo EL Nikkor an apochromat. YES. I am afraid that the word Apochromat has become somewhat of a marketing tool, open to wide interpretation, and which appears to now mean reduced secondary spectra by those who market their lenses.

[Bob Salomon]

Sorry Pat your definition of Apo is not correct (unless you are talking about microscopes)

First problem is that a point has size so when you say the rays are brought to focus at a common point how large is that point?

Although this has long been a simplified way of describing an apo lens it is not what the photographic lens manufacturers use nor is it used by the Association of Optical Industry Purchasing Agents.

A more precise definition is that the lateral chromatic aberrations of the secondary spectrum are reduced to within a minute percentage of the focal length of the lens. This describes the point that size that you mentioned in your post.

Yes the Apo Sironar S, N and W as well as the Apo Ronar, Apo Sironar Digital, Apo Macro Sironar, Apo Macro Sironar Digital, Apo Sironar, Apo Rodagon N and Apo Rodagon D as well as Schneider's apos all are apochromatic.

But the real proof is not worrying about how many angels can dance on the head of a pin but what are the results photographically.

[Patrick Raymore]

Open quote " The term superchromat implies that at least four wavelengths are brought to a come focus, where as the term apochromat indicates that three wavelengths are corrected." end quote MODERN LENS DESIGN by Warren Smith 1992 page 173.

Open quote " Apochromat: A lens in which light of three colors is brought into common focus" end quote PHOTOGRAPHIC OPTICS by Arthur Cox 15th edition 1974

Open quote " True apochromatic correctionthree images of identical size for three spectra lines." End quote. APPLIED PHOTOGRAPHIC OPTICS by Sidney Ray 1988 1st edition page 97

Pretty plain talk don't you think. Every optical book on my shelf says the same thing, some in more technical or complex terms, but the definition remains the same. Kingslake, Shannon, Cook, Cox, Smith etc, they all say the same.

Now lets get specific.

Example: the Apo Rodagon. (Now don't think I a picking on Rodenstock, Bob, there is plenty of other examples that I could finger.) When the Apo Rodagon came out around 1979 I was very excited. An apochromat at a reasonable price? After getting this lens my findings were the same as countless objective observers.

Open Quote "The Apo Rodagon isn't an apochromat. Now don't get me wrong, it is a great lens and may be my first choice. But Rodenstock should never have called it an apochromat. It is a great lens anyway. But in my test fringing was visible even wide open, though it cleared up some color on stopping down. Great lens, yes. Apochromat, no." End quote by columnist KRAMER from MODERN PHOTOGRAPHY (vol#43 issue #10 1979)

Since then these findings have been confirms by many authors. Are these findings trivia and inconsequential? I don't think so. Read POST EXPOSURE by Ctien 1997. He devotes 1/4th to a 1/3th of his book to consequences to lenticular chromatic aberration and variable contrast papers. And by the way, Ctien reconfirms that the Apo Rodagon is not apochromatic.

As for the Sironars. the Chromatic graphs for these lenses, as represented in Rodenstock's own brochures, clearly identify these lenses as achromats (not aopchromats) with a reduced secondary spectra. Only two wavelengths are brought a common focus. Compare these chromatic graphs with that of an apochromat from any optical book (example: THE ART AND SCIENCE OF OPTICAL DESIGN by Robert Shannon 1997 page 191) and you will see that these lenses are clearly not apochromatic.

I am proud of Nikon, who appears reluctant to label a lens apochromatic just because they used low or anomalous dispersion rare earth glasses to reduce its chromatic aberrations. The Nikon T series is a good example. This restrain flies in the face of commercial pressures.

Bear with me for a few more comments.

It is not necessary or even desirable for a lens to be apochromatic to be an excellent lens. In fact, form a design point view, it is more important for a designer to correct chromatic variations of spherical and astigmatic aberrations, than to go chasing the reduction of the secondary spectra, especially in the shorter focal lengths. This is not to say that the secondary spectra are inconsequential. In Tele designs, long focal lengths, macro work etc attention needs to be paid to the secondary spectra if you want the result to be an excellent lens, and no number of dancing angels will help will help you with this.

For those who want to indulge themselves further, an excellent mathematical exploration of subject is contained in the book, A SYSTEM OF OPTICAL DESIGN by Arthur Cox 1964 Appendix G. For those us with a little computer skill, a demo version of the sophisticated optical design program ZEMAX is available from many shareware sites. These programs (ZEMAX, CODE V etc) are delightful to play with.

And take the commercial brochures with a bucket or two of salt. Calling a lean pig a bird just doesn't fly with me.

[Bob Salomon]

Sorry Pat,

You are still incorrect.

The D.I.N. definition of an Apo lens is as I stated:

The lateral chromatic abberrations of the secondary spectrum are corrected to within a percenyage of the focal length of the lens.

This is the definition that photographic lens manufacturers manufacture to.

On the other hand Kramer, Ctein et al are not lens manufacturers. nor are the lens designers, nor have they been either at any time.

Kramer was the advertising agency for Nikon and Ctein is an excellent printer. But neither are optical engineers.

i would be happy to mail you industry definitions from several sources that describe an apochromatic as exactly as I have described above.

Unfortunately the definition that you are using is not what any lens or camera manufacturer uses.

[neil poulsen]

"Unfortunately the definition that you are using is not what any lens or camera manufacturer uses."

Two counter-examples: Kingslake certainly, and arguably Wisner, are two lens manufacturers who define apo-chromat as Mr. Raymore suggests.

[Bob Salomon]

Wisner does not make lenses. He sells sets of Schneider lens elements from the G Claron and re-labels them.

Kingslake also does not work for a lens manufacturer.

[Struan Gray]

Bob: how old is that DIN spec? My impression was that it is a recent definition introduced because so many manufacturers were already 'misusing' the APO term in marketing literature (mostly for 35 mm lenses). The motivation for setting up the DIN/ISO standard was truth in advertising, not optical engineering per se.

Pat's is the classic definition, which I think goes back to Abbe himself - hence the reference to microscope objectives. It's certainly the one in the late C19th optics textbooks we have in the history section of our physics library, as well as all the modern texts Pat cited.

The 'problem' with the classic definition is that is says nothing about the lens' performance at wavelengths other than the three which have a common focus. If you sum aberrations over all visible wavelengths, a non-APO lens can in principle have better performance than an APO design. The 'new' definition concentrates on what is important for practical colour photography in daylight: total colour errors across the field.

That said, the APO designation is almost useless except as a one-word marketing tool. APO lenses in the large format world tend to be excellent, but so do non-APO lenses. The differences are subtle, and tend to revealed clearly only in specific situtations. If you care about such minutea you have to test the lenses yourself.

[Pete Andrews]

It seems that the term "Apo" has been de-valued lately, possibly by the introduction of the term to the mass-market by independent 35mm lens manufacturers. I am surprised however that suppliers of professional large format lenses would use a looser definition of the word than has been accepted in the past. i.e. corrected for 3 wavelengths.

Perhaps it's because the traditional market for these lenses in the field of repro-graphics has been reduced to near zero by new technology, and there is very little demand for professional large format work these days, that former well-respected names in the optical industry have been reduced to clutching at straws to gain a sale.

[neil poulsen]

Kingslake was in charge of the Lens Design at Kodak for 31 years, during the period when they produced and improved the Commercial Ektars. Wisner involves himself in the mfg. process, since he recells and recements Schneider elements to optimize them for convertible use. (Hutchings, Gordon (1998). The Wisner Convertible Plasmat Lens, View Camera, Jan/Feb, 1998.) On his knowledge of lenses, Kingslake is in a class by himself, at the top. Wisner's knowledge is extensive.

What I notice in this discussion is that Mr. Raymore's definition expresses an ideal: all three wavelengths will come to focus at the same point along the lens axis. While by no means an expert of any sort, based on what I've read, lens design is a balancing act between the different aberrations. So, such a definition must be operationalized for practical use, which usually means expressing the definition to within a delta or a percentage.

I'm wondering if the DIN standard referred to above is an honest attempt at this, or if it's a slight of hand that allows manufacturers to call their lenses apo's, when in fact, they may not meet this criteria. Manufacturers can influence standards to those that serve their purposes.

QUESTIONS: What is the actual percentage used in the DIN definition? Is "lateral" along the lens axis? Does the "secondary spectrum" include colors other than the two wavelengths that must be brought into common focus to achieve an achromatic lens. (Vs. an apo-chromatic lens.)

[Pete Andrews]

"Lateral", as in lateral colour, means at right angles to the optical axis in the plane of focus. It fundamentally indicates a difference in image size for different wavelengths of light. Secondary spectrum includes all colours not brought to a common focus.

The term focus is, however, open to debate.

Incidentally, why is the Plasmat cited as an eponymous lens? The Rapid -Rectilinear (1866), as progenitor of the symmetrical family of lenses I can understand. Also the Planar as the first double-gauss type, but surely the other archetype of the symmetrical lens style must be the Goerz Dagor (1893), not the much later and little used Plasmat?