Ads for telelenses frequently highlight the use of "low dispersion", "ultralow dispersion", "ED" etc. glass which are used (mainly?) to lower chromatic abberations. As dispersion seems (technically) related to the refractive power, I would like to know what counts as a "low dispersion" glass. An old sheet from Schott shows glass with refractive indices ranging from 1.44 to 1.96 with Abbe numbers (as a measure of dispersion) ranging from ca. 90 to 20, with a correlation that high refractive glass usually has low Abbe numbers (=high dispersion). I assume that -especially for telelenses- designers like high refractive glass to keep curvature and thickness small, and, to compensate for the high dispersion of these glasses, to select those with a relatively high Abbe number. Thus just to say that a glass is "low dispersion" seems irrelevant to me as long as the correlated refractive index isn't mentioned. So my question in brief:

What are vd AND nd values of those glasses which justify to be named "(ultra-)low dispersion"? (vd and nd = Abbe number and refractive index at 588nm)

-- Konrad Beck (K.Beck@ibcp.fr), August 22, 2000

Answers

"Thus just to say that a glass is "low dispersion" seems irrelevant to me as long as the correlated refractive index isn't mentioned" This is exactly what I was talking about in the post before your. There are no standards set by the industry for what is ED or APO or LD glass. My favorite is a lens I saw for sale described as a "semi-apochromaric" design. the marketing people are playing around here, becasue people really go for these labels. I think you can be sure if Leica calls a lens an APO lens, it is, but I have to laugh at some of the $250 consumer grade zooms claiming LD performance.

-- Andrew Schank (aschank@flash.net), August 22, 2000.

I don't think there is any detectable standard regarding what combination of characteristics is needed before it is acceptable to give optical glass a commercially attractive name. Also, I don't think there is such a thing as APO glass. APO refers to the ability of a lens to focus several different parts of the visible spectrum on one plane. That requires at least 2 pieces of glass with different indices of refraction. Also, I don't think APO performance necessarily require any special glass. Right combinations of normal crown and flint glass can probably make lenses of certain focal lengths apochromatic.

-- Chuck Fan (chaohui@msn.com), August 22, 2000.

Brief answer to question:

BK 1.52 62 normal dispersion FK52 1.486 81.8 Schott ED CaF2 1.433 94 Calcium Fluoride ED

To comment on other responses: APO (apochromatic) means that 3 wavelengths are brought to within a certain amount of a common focus plane. This can be done with fairly common optical glass if you're willing to compromise on on other characteristics, usually speed. The old process lenses were like f9 or f14 and were actually used at f32-64.

Pretty much all glasses follow a similar vd/nd curve. Some materials have anomalous dispersion where their vd/nd is off the curve to the left. The best example is Fluorite (Calcium Fluoride). Some other glasses approach this by adding various rare earths. I understand that the new 'ED' glasses are proprietary mixtures of glass, rare earths and Fluorite; only the manufacturer knows for sure.

-- Duane K (dkucheran@creo.com), August 22, 2000.

Thanks, especially to Duane K. Seems that my assumption that telelens designers would prefer high refracting glass is wrong. According to your numbers, and especially mentioning CaF2, ED/LD seems indeed to refer to the high vd, even if this means a low nd. As CaF2 in the ads seems still worth to mention specifically, I would conclude that the LD/ED glasses have a vd<94. Though small number differences are certainly important, the differences between vd=82 (BK52), 84 (BK51), 90 (BK54) and 94 (CaF2) in which range probably the more modern proprietary mixtures fall, don't look like a "huge" advancement to me. Or have I gotten something (more) wrong?

I don't think there is any detectable standard regarding what combination of characteristics is needed before it is acceptable to give optical glass a commercially attractive name. Also, I don't think there is such a thing as APO glass. APO refers to the ability of a lens to focus several different parts of the visible spectrum on one plane. That requires at least 2 pieces of glass with different indices of refraction. Also, I don't think APO performance necessarily require any special glass. Right combinations of normal crown and flint glass can probably make lenses of certain focal lengths apochromatic.

-- Chuck Fan (chaohui@msn.com), August 22, 2000.

Brief answer to question:

BK 1.52 62 normal dispersion FK52 1.486 81.8 Schott ED CaF2 1.433 94 Calcium Fluoride ED

To comment on other responses: APO (apochromatic) means that 3 wavelengths are brought to within a certain amount of a common focus plane. This can be done with fairly common optical glass if you're willing to compromise on on other characteristics, usually speed. The old process lenses were like f9 or f14 and were actually used at f32-64.

Pretty much all glasses follow a similar vd/nd curve. Some materials have anomalous dispersion where their vd/nd is off the curve to the left. The best example is Fluorite (Calcium Fluoride). Some other glasses approach this by adding various rare earths. I understand that the new 'ED' glasses are proprietary mixtures of glass, rare earths and Fluorite; only the manufacturer knows for sure.

-- Duane K (dkucheran@creo.com), August 22, 2000.

-- Konrad Beck (K.Beck@bcp.fr), August 22, 2000.

Sorry for not deleting the previous answers in my new entry. Had them copied to just see them during my writing.

-- Konrad Beck (K.Beck@ibcp.fr), August 22, 2000.

I'd like to ask another question in relation to this and the previous post I posted. Would you be able to tell the difference between a picture taken using a lens with a low dispersion glass and another using a lens without, assuming that the same setup and technique were used for both shots? If so, what would it be?

-- Ron Gregorio (gregorio@ksc.th.com), August 23, 2000.

Regarding your second question:

The non-APO lens would show more "color fringing". By "color fringing", I mean when you can see a bit of blue or red along edge of object. It's usaully very obvious when you see colors around the edges of black lettering on a white sign.

-- Geoffrey S. Kane (grendel@pgh.nauticom.net), August 23, 2000.

I find this question interesting concerning the APO designation. People have quoted me words to the effect that "all Canon L lenses are apochromatic". But I say it all depends on what your definition of the term apochromatic is. Leica apo terminology mean that the "apochromatic characteristics" (whatever this means?!) must be visible at full aperture - and, as a Leica user, I have the utmost respect for Leica Apo glass - but they do not pretend that, say a 50mm Summicron is apochromatic or the 80mm f1.4. Then there are Sigma Apo lenses - I personally doubt they are of the same standard as the Leica apo equivalents (e.g. 300mm Sigma 2.8 apo versus the 280 2.8 Apo from Leica). Does any one have any idea of what standards are used for the term "Apochromatic" or is it really just an only slightly useful label of supposedly "superior" performance? Out of prejudice I think that the Leica label of Apo or the Zeiss "Superachromat" designation does mean something specific rather than just being a marketing label, but no one yet has told me what it is exactly apart from bringing red, green and blue light to focus on the same plane, which seems to be so elementary as to be unhelpful. Why is it that we don't know what it really means in camera lens terms?

-- Robin Smith (rsmith@springer-ny.com), August 23, 2000.

JCII (Japan Camera and Optical Instruments Inspection and Testing Institute --you know the guys that put those litle gold stickers on Japenese cameras) probably had a standard for APO, but I have no clue what it might be.

Nikon never called any lenses "APO". They just let you know which lenses have "ED" glass. With some lenses the ED versions are great improvements (like the 50-300 f4.5), with others the ED version may be better, but there's nothing really wrong with the non-ED model.

-- Geoffrey S. Kane (grendel@pgh.nauticom.net), August 24, 2000.

Just take any lens brochure and it will tell you what APO is for. Light goes through glass, refraction happens, colors are separated (like a rainbow), picture shows color fringing. APO is more useful in telephoto lenses because the light has to travel a longer distance from the time colors are separated to the film. By the time the light reaches the film, the colors are spread all over. To illustrate this: walk away from a friend at say, an angle of 30 degrees. Walk for 20 feet and your still close to your friend, walk a mile and you'll be far. Manufacturers use Extra-Ultra-Super-Dupper-low dispersion glass to minimize refraction (I think), thus color fringing.

I have no background in optics, only high school physics. I just used common sense to come with this explanation. I could be totally wrong so if someone REALLY knows for sure, please say so. What I don't have any clue about is wether there are industry standards (sorry Konrad) and what the actual design of the lens (shape,molding) has to do with chromatic abberation.

-- Erick Lamontagne (ericklam@globetrotter.net), August 24, 2000.

http://www.ohara-inc.co.jp/ohara_e/n2_00/hinmoku/kougaku/f3_11.htm

check that for what is considered "low dispersion" glass type and note that it have nothing to do with "APO". Also note that there are various types on chromatic aberration and usually being only means that the "longitudinal" chromatic aberration is corrected for 3 or more wavelengths. There is no bound on what might happen at the other wavelengthes or other types of chromatic aberration.

-- idiot (idiot@aceshardware.com), November 30, 2000.