I'm curious as to how apochromatic correction in a lens is achieved.

Let me use two examples of highly regarded lenses, the Zeiss ZA 135/1.8 Sonnar, and the Voigtlander 125/2.5 APO macro. The Zeiss contains 2 ED elements, and is by all accounts very sharp, yet is clearly not APO corrected. The Voigtlander also contains 2 ED elements and by all accounts is truly an APO lens. Part of the recipe for an APO lens is the use of glass that is either ED, UD, AD, or the use of Crystalline Flourite, yet it seems clear that this alone is not sufficient for APO correction. What are the other factors involved? How was Voigtlander able to achieve such a highly corrected lens for such a modest price ( current prices not withstanding, it sold for about $600 new)?

The glass types you mention certainly help a lot. The rest is all up to the design of the lens; use of compensator groups, aspherical surfaces (not necessarily in the CV 125), etc.

Here's a recent thread that covers all you ever wanted to know about APO, and more - a lot more.

http://www.fredmiranda.com/forum/topic/795591

Here's another full of beautiful photos, and less distracting talk.

http://www.fredmiranda.com/forum/topic/810600

I read that thread, but there's no discussion about how APO correction is achieved. I'm not concerned with "absolute scientific APO", but rather lenses that exhibit APO performance in practical situations with today's cameras. That's why I mentioned the Voigtlander 125/2.5 APO. From every example I've seen, it is for all practical purposes an APO lens, even wide open. That Cosina accomplished this in a $600 lens is impressive.

Indeed.

I think the main reason you don't see most of the major manufacturers making APO or near-APO lenses is that it's not something the general public looks for when selecting a lens. By not striving for apochromatic lenses they are more free during the design process to prioritize and reduce other lens aberrations. I don't see the CV125 being impressive for being APO for cheap (at least before it was discontinued and prices shot way up), I see it being impressive as being cheap, APO AND without compromising performance (that we're aware of). I would have bought one if I'd been aware of it while it was still in production, as I'm sure many here would.

I'm not sure there's many if anyone on this board who has enough optical design experience to tell you exactly how to acheive apochromatic lenses.

One thing that I would just like to add, for no real reason, but I have a pretty good hunch that telephotos are much easier to design and also easier to design apochromatically. Mostly since they don't require lenses with such high indexes of refraction.

I don't know how they are designed, but I remember awhile back when the ZF/ZE 21mm was announced and the MTF for it was posted the guy who designed the Coastal Optics APO 60mm macro made a bunch of wonderful comments on the MTF. He obviously knows how to achieve APO correction. If we are lucky maybe he will post and educate us a bit on how it is done. And to add to Valorin's comments based on the lenses that are APO it would seem that not only are longer lenses easier to make APO, but also slower lenses, and perhaps macro lenses as there seems to be a lot of APO macros.

Valorin wrote:
One thing that I would just like to add, for no real reason, but I have a pretty good hunch that telephotos are much easier to design and also easier to design apochromatically. Mostly since they don't require lenses with such high indexes of refraction.

I also think it's more important in telephoto designs than in wide angle designs.

You can design a lens with many ED glass elements without being apochromatic.

Some manufacturers designate their lenses as APO only when the performance meets the scientific definition of being apochromatic.

Some other manufacturers designate their lenses as APO as a marketing term, meaning some exotic ED glass elements are used.

Some manufacturers most notably Zeiss does not advertise heavily or sometimes not even tell you exotic glass elements are used. They expect the lens' real world performance to speak for themselves.

The scientific definition of an apochromat can be found here

http://www.dantestella.com/zeiss/achromat.html

surfotog wrote:
I'm curious as to how apochromatic correction in a lens is achieved.


The techincal term Apochromatic denotes a particular degree of correction of Chromatic and Spherical Aberration, and Coma. The marketing term APO hints at achieving the technical term in some undisclosed conditions.

Probably more germane to the intent of your question, CA is corrected by exploiting the same phenomenon that causes it in the first place - dispersion.

We're all familiar with refraction; it's what makes a yardstick half submerged in a swimming pool appear to be bent in the middle. But not all of the colours in white light will be refracted to the same degree as they cross into a material, and this is what causes the dispersion. A prism is the clearest demonstration.

However, not all materials refract and disperse light to the same degree. By combining elements of low and high indexes of refraction, but matched dispersion when ground to opposing power, a second element can cancel the dispersion of the first, without cancelling all of the (desired) refraction. Materials with low dispersion to start with make this process easier.


Actual lens designs are concerned with correcting much more than just CA, so elements are ground to walk, chew gum, pat head, rub tummy, and do calculus, all at the same time.

/Acme

I wish I could be OlyAcme for one day so I could have an insight in how it would feel to know and understand so much about the technical side of optics and still be able to explain it on a level that I can grasp.

cheers! Henrik

olyacme,

Thank you for that explanation. Even my aging, scientifically untrained mind was able to grasp it :~)

olyacme wrote:

However, not all materials refract and disperse light to the same degree. By combining elements of low and high indexes of refraction, but matched dispersion when ground to opposing power, a second element can cancel the dispersion of the first, without cancelling all of the (desired) refraction. Materials with low dispersion to start with make this process easier.

/Acme

Thanks Acme. This is what I meant by "compensator group". You've explained it well

google is our friend; so is wikipedia:

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

shows how lenses are arranged to acheive apochromatic correction.

According to Dr.Ernst Abbe (founder of Zeiss), the term 'APO' means three things:
1.correction of secondary spectrum.
2.convergence of three colors.
3.high order achromatism.

Optically, there are 4 levels of chromatic correction - Achromats, Semi-apochromats , Apochromats , Super-achromat. So far, only Zeiss makes super-achromat lenses. I don't know if the Coastal Optics APO 60mm macro should be considered as a SA lens. Here is the explaination of Apochromat from wiki, and I think it's the best answer I've ever seen.

"Apochromatic designs require optical glasses with special dispersive properties to achieve three color crossings. This is usually achieved using costly fluoro-crown glasses, abnormal flint glasses, and even optically transparent liquids with highly unusual dispersive properties in the thin spaces between glass elements. The temperature dependence of glass and liquid index of refraction and dispersion must be accounted for during apochromat design to assure good optical performance over reasonable temperature ranges with only slight re-focusing. In some cases, apochromatic designs without anomalous dispersion glasses are possible."

"how apochromatic correction in a lens is achieved"...? :-)

I some cases, the explanation is really easy. Build a symmetrical lens (optical construction looks the same, only mirrored, on both sides of the aperture - and is generally well corrected for the usual abberations) - and only use it in 1:1 macro work! :-)

Otherwise, the "how" is not something that's explainable in a space as cramped as this, you'd need a good many chapters in a book to make the different ways even remotely explainable. As OlyAcme pointed to earlier (indirectly), the problem lies with all the other corrections the lens has to make. The amount of "bending" of the light has to be assymetrical in/out if you want to be able to focus on stuff thats not at an 1:1 distance from the lens...

Think of the lens (in the most simple, easy way) as two parts - as long as we're talking about primes! - one is leading IN to the lens from the front, one is leading OUT from the lens towards the sensor/film. A symmetrical 1:1 makro lens can be constructed with exactly the same amount of "bending" and surface curvatures on both sides. A lens that's meant to focus on a longer distance from the camera [than the lens - film plane distance] has to use assymetrical constructions. An assymetrical construction bends light LESS on IN-part (focus plane is further away), and MORE on the OUT part (distance to film plane is roughly the same as in the 1:1 example, but shorter!).

This makes the construction off-balance when it comes to many things, and among them is the colour dispersion.The colour dispersion effects (CA of all sorts and orders) are generally not the first on the list of "things to correct" for a lens designer. People generally want "general" sharpness and freedom from SA and distortion before the "almost perfect colour correction" arrives on the agenda.

To put things in perspective, I like to cite this little list of facts...:
*Different glass types will bend light of different wavelengths by a different amount.
*How far the colours are allowed to "separate" geometrically before hitting the next boundary (glass/air or air/glass) is determined by the distance it travels up to that next surface.
*geometries and distances are very constricted if you want "sharp" and "distortionless" pictures with a certain lens formula
*The 125F/2.5 lens has an apparent aperture area of almost 2000mm^2
*A normal pixel in a digital camera has an area of 0.000036mm^2
*The amount of "concentration" of light passing through the aperture then has to be 2000/0.000036 = 5.5 million times if you want a pixel-to-pixel sharp picture in the end...

The CO60 is not a "superachromat" AFAIK, as it only has three zero-passing points in the LoCA chart. The difference is mostly academical though, as the achromatic behaviour of the CO lens is so wide that the difference from "perfect" within the visible light spectrum can indeed be less than that of a "superachromat" lens.
I've only handled a superachromat once, a Zeiss 300F/2.8 for hasselblad. Probably the most expensive singular piece of photographic equipment that I've ever touched... :-) The owner and I jokingly had a conversation about how he would prefer the package to land on the back, and totally destroy the 35k$ HB DMF back, than that the Zeiss should even touch the ground... It's a LOT more expensive (and a lot more rare) than the HB3D it was mounted on.

phuang3 wrote:
Here is the explaination of Apochromat from wiki, and I think it's the best answer I've ever seen.

"Apochromatic designs require optical glasses with special dispersive properties to achieve three color crossings. This is usually achieved using costly fluoro-crown glasses, abnormal flint glasses, and even optically transparent liquids with highly unusual dispersive properties in the thin spaces between glass elements. The temperature dependence of glass and liquid index of refraction and dispersion must be accounted for during apochromat design to assure good optical performance over reasonable temperature ranges with only slight re-focusing. In some cases, apochromatic designs without anomalous dispersion glasses are possible."

But, except for the first sentence, this has nothing to do with the definition. It's just a bunch of technical babble.

Abbe coined the term, so it seems appropriate to use his definition, as relayed by Thomas Back:

Apochromat: an objective corrected parfocally for three widely spaced wavelengths and corrected for spherical aberration and coma for two widely separated wavelengths.

Notice how, unlike the marketing definition, this one is is talking about more than just CA. I can't stress enough how critical this is. It's entirely possible to produce optics that have negligible CA yet are poor for imaging. It's cheap and easy from a design standpoint to trade SA for CA or vice versa. But just because SA isn't as "in your face" as CA doesn't mean it's any less destructive to a good image.

At any rate, rehashing the definition of Apochromatism doesn't seem to have been TOPs intent for this thread.

/Acme

Thanks to all for the replies, this is very informative.

A related question; Are there any books on lens design that can be grasped by a layperson of reasonable intelligence ?

I'm guessing that any optics book or book on lens design is going to have some math in it. So it probably depends on how much of a layperson you are. If you feel comfortable with college level math/physics, then there are probably some books out there. I don't know for sure. I've never formally studied optics, but the optics book I have looks like you need to be comfortable with multivariable calculus and tensors. Of course, this isn't a book on lens design, but intro optics aimed at physicists (Introduction to Modern Optics by Fowles, $10 new).

That being said, I saw a couple photographic optics books on amazon that might be interesting reads. For $100ish.
Applied Photographic Optics and History of the Photographic Lens. And The Photographic Lens.

Rather than looking for anything with "optics" in the title, which will probably go heavy on the math, you might try looking for some of the amateur telescope making books. These tend to focus (no pun intended) on mirror designs, but will also run through the basics for Achromatic refractors and eyepieces. There will still be some math, but it will be applied in cookie cutter formulae.

OTOH, if you've got time and money to burn, "Handbook of Optical Systems" might be the reference to have.

... or get your Ph.D in photonics ;-) (DOH!!!!)

D