What is ED Glass? (1 Viewer)

AlanFrench · Nov 24, 2006

To an optician the term ED means Extra-low Dispersion. As you know, if you make a prism and pass light through it, it will break white light up into its component colors. The blue end of the spectrum is bent the most, and the red end is bent the least. The prism disperses or spreads out the colors that make up white light.

Dispersion is a simply a measure of how much the colors are spread out by the prism. A typical achromat is made of a low dispersion crown glass and a high dispersion flint glass. The recent advent of extra low dispersion materials, often called fluor-crowns, allows opticians to make two element lenses from a fluor-crown glass and a crown glass.

People often mistakenly believe the superior color correction of such doublets is because of the extra-low dispersion. That is incorrect. You could have an ED material that would provide absolutely no such advantage. Neither is such material classified as "abnormal dispersion" because of its extra-low dispersion. There is another characteristic that allows ED material to be used for better color correction (with the proper matching glass or glasses), and caused it to classified as "abnormal dispersion."

I'll leave it at that for now. In the meantime, see if you can find a glass diagram on the web (or look at the next post). Take a look at where the best ED glasses lie in relation to the crown glasses. Also note where the flint glasses lie in relation to the crown glasses. Keep in mind that designers want two glasses with substantially differing dispersions. Another hint - Ohara's FPL53 and BSM81 can be combined to make an absolutely superb apochromat, one that even meets James Baker's very stringent criteria.

I should mention that the horizontal axis of a glass diagram or glass map is the dispersion. Opticians usually use the reciprocal of disperson, so higher numbers mean lower dispersion. The ED material is usually to the left, and the flints are way to the right. FPL53 and CAF2 (fluorite) have a dispersion of 95.

Clear skies, Alan

Please see next post....

AlanFrench · Nov 25, 2006

I had forgotten that you are most likely to turn up diagrams of dispersion versus index of refraction (Abbe diagram), which is not what you want, so I'll make it easier. Here is the diagram, with FPL53 and BSM81 labeled.

A traditional achromat would use BK7, with a dispersion, Vd, of 64.17, and F2, with a dispersion of 36.37. From these values you can see about where the two glasses would lie. Remember that FPL53 and BMS81 can produce a very fine apochromat.

The designer must pick glasses with substantial differences in dispersion, for ease of manufacture, good control of aberrations, and reasonable focal length.

Clear skies, Alan

AlanFrench · Nov 25, 2006

Here is a new diagram, with labels added for BSL7 and TIM2, the Ohara equivalents of BK7 and F2 and the standard glasses for achromats. The red dot to the left of the TIM2 label is the glass.

Clear skies, Alan

henry link · Nov 25, 2006

Alan,

Thanks for posting the diagrams. I would just add that in these diagrams the best match results when the horizontal distance between the glass types is as large as possible and when a line drawn between them is as close to horizontal as possible. You can easily see how well FPL53 and BMS81 fulfill both requirements and how very far from horizontal a line drawn from BSL7 and TIM2 would be.

Henry

AlanFrench · Nov 25, 2006

henry link said:
Alan,

Thanks for posting the diagrams. I would just add that in these diagrams the best match results when the horizontal distance between the glass types is as large as possible and when a line drawn between them is as close to horizontal as possible. You can easily see how well FPL53 and BMS81 fulfill both requirements and how very far from horizontal a line drawn from BSL7 and TIM2 would be.

Henry

Henry,

Exactly. The vertical axis is call partial dispersion, and it tells how dispersion varies with color. To get perfect color correction, you need glasses that have the same partial dispersions.

The line between BK7 (~BSL7) and F2 (~TIM2) is called the Abbe line, and the majority of glasses lie along this line. Since the slope of the line connecting the two glass choices determines the color correction, you can see that crown and flint combinations are all essentially the same. Some lenses have been made with glasses that differ little in dispersion to get better color correction, but such a lens is not for the faint hearted, and not at all suitable for regular production.

The ED glasses are called "abnormal dispersion" because they are well of the Abbe line. If they had remained along the Abbe line (smaller partials), they would have been no help in improving color correction.

Clear skies, Alan

greg g · Nov 28, 2006

AlanFrench said:
Henry,

Exactly. The vertical axis is call partial dispersion, and it tells how dispersion varies with color. To get perfect color correction, you need glasses that have the same partial dispersions.

The line between BK7 (~BSL7) and F2 (~TIM2) is called the Abbe line, and the majority of glasses lie along this line. Since the slope of the line connecting the two glass choices determines the color correction, you can see that crown and flint combinations are all essentially the same. Some lenses have been made with glasses that differ little in dispersion to get better color correction, but such a lens is not for the faint hearted, and not at all suitable for regular production.

The ED glasses are called "abnormal dispersion" because they are well of the Abbe line. If they had remained along the Abbe line (smaller partials), they would have been no help in improving color correction.

Alan,

This very interesting yet a little over my head. On a chart like this can these five different birding scopes be marked in relation to this information; Swarovski 80HD, Leica 77APo, Ziess 85, Nikon 82ED, Kowa TSN 823 would be with a 30x wide and a 20x60 zoom? Would something like that tell us anything on manufacturer quality and the scope quality?

AlanFrench · Nov 28, 2006

greg g said:
AlanFrench said:

Henry,

Exactly. The vertical axis is call partial dispersion, and it tells how dispersion varies with color. To get perfect color correction, you need glasses that have the same partial dispersions.

The line between BK7 (~BSL7) and F2 (~TIM2) is called the Abbe line, and the majority of glasses lie along this line. Since the slope of the line connecting the two glass choices determines the color correction, you can see that crown and flint combinations are all essentially the same. Some lenses have been made with glasses that differ little in dispersion to get better color correction, but such a lens is not for the faint hearted, and not at all suitable for regular production.

The ED glasses are called "abnormal dispersion" because they are well of the Abbe line. If they had remained along the Abbe line (smaller partials), they would have been no help in improving color correction.

Alan,

This very interesting yet a little over my head. On a chart like this can these five different birding scopes be marked in relation to this information; Swarovski 80HD, Leica 77APo, Ziess 85, Nikon 82ED, Kowa TSN 823 would be with a 30x wide and a 20x60 zoom? Would something like that tell us anything on manufacturer quality and the scope quality?

Click to expand...

Greg,

Unfortunately, the folks making scopes are not telling what glasses they are using, so we can't look at them on the chart. Also, things get more complicated when you move on the lenses with more elements. You can see that there aren't too many abnormal dispersion glasses available, although other glass makers like Scott and LZOS have glasses similar to the Ohara versions.

While I find optics interesting, the most important question is whether a scope works well enough for me or you. I think most folks can make that decision without knowing the choices the manufacturer made in the scope. Some folks with an extreme interest in optics find themselves looking for aberrations or problems with the optics, perhaps to the detriment of their enjoyment. I try to concentrate on the beautiful coloring of the Red-bellied Woodpecker's head, and things like that.

Clear skies, Alan

iporali · Nov 28, 2006

What about triplets?

AlanFrench said:
A typical achromat is made of a low dispersion crown glass and a high dispersion flint glass. The recent advent of extra low dispersion materials, often called fluor-crowns, allows opticians to make two element lenses from a fluor-crown glass and a crown glass.

Alan,

What kind of glass is the third lens used in APO-triplets? I think I have read/heard that Leica (?) would have some secret glass formula for this glass. Do they utilize those "earth metals" (Lanthanum), which make high refraction and low dispersion?

Ilkka

hinnark · Nov 28, 2006

iporali said:
Alan,

What kind of glass is the third lens used in APO-triplets? I think I have read/heard that Leica (?) would have some secret glass formula for this glass. Do they utilize those "earth metals" (Lanthanum), which make high refraction and low dispersion?

Ilkka

Leica declares the use of FL-Glass (glass with included flouride ions, not calcium flouride) with their Apo scopes. I don´t think that they have secret formulars of glass because Leica don´t make glass. However, the Apo Televids are surely among the best working optics available in respect of color correction.

Steve

iporali · Nov 28, 2006

hinnark said:
I don´t think that they have secret formulars of glass because Leica don´t make glass.

OK, Thanks Steve - this is what I suspected, because the term "APO-glass" is often incorrectly used as meaning the whole complex of ED-containing doublet or triplet. I just thought if Leica had some special glass, it could be this "third element" whatever it is.

Ilkka

hinnark · Nov 28, 2006

iporali said:
OK, Thanks Steve - this is what I suspected, because the term "APO-glass" is often incorrectly used as meaning the whole complex of ED-containing doublet or triplet. I just thought if Leica had some special glass, it could be this "third element" whatever it is.

Ilkka

I found S. Ingrahams explanation quite useful:

http://www.zbirding.info/zbirders/blogs/sing/archive/2006/08/08/181.aspx

BTW the only manufactorer of binoculars who makes his own glass is AFAIK Nikon.

Steve

iporali · Nov 28, 2006

hinnark said:
I found S. Ingrahams explanation quite useful

This is a very nice article, but I am sure it could be even improved by having eg. Alan proofread it :t:.

Ilkka

AlanFrench · Nov 28, 2006

iporali said:
This is a very nice article, but I am sure it could be even improved by having eg. Alan proofread it :t:.

Ilkka

Ilkka,

Thanks. I appreciate the compliment.

I did a quick reread of the article, and the one thing that stuck out was the incorrect idea that it is the low dispersion that allows ED material to work its magic.

I do wish the colored lines corresponded with the correct colors in the first diagram (blue focuses closest, then yellow, and finally red).

I think I may have to borrow my wife's writer's hat.

Clear skies, Alan

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