ED glass in Binoculars

[kristoffer]

So, lets discuss it. What is it? How does it work? Is it always the same thing with different names depending on manufacturer?

[henry link]

This is a complex and technical subject. Better for anyone with a real interest to start at the beginning with a an optics text like Rutten and van Venrooij, "Telescope Optics". There you can learn the basics of how glass types are chosen and matched to correct chromatic and other aberrations. There is probably similar information online.

There are also many excellent posts at astromart.com forums by virtuoso telescope designer and maker Roland Christen on the subject of ED glass types. Going through his archived posts is an optics education, but unfortunately you have to pay $12 to join Astromart.

Ultimately, however none if that information will tell you what you really want to know about a particular ED binocular. Even if you have lens designing software on your computer you will never be given enough information about the glass types and design of any ED binocular to make a good prediction about it's performance. So, let me modestly recommend some tests shown on the thread below which will show you exactly how well chromatic aberrations are corrected in any binocular.

http://www.birdforum.net/showthread.php?t=134310

Henry

[Kevin Purcell]

And of course search for Abbe number here ... most of the threads that have said something interesting have abbe number in them!

[FrankD]

Thank you for the suggestions Henry. I do have a subscription to Astromart but I have, honestly, only been there once or twice. I will give it a go shortly though.

Some information is better than nothing at all.

[ThoLa]

So, lets discuss it. What is it? How does it work? Is it always the same thing with different names depending on manufacturer?

there is a very insightful essay here:

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/3193178/page/0/view/collapsed/sb/5/o/all/fpart/1



so there is nothing much to discuss.
let's stop the hype.

[kristoffer]

The first reply stated that it is too complicated subject to discus for laymen's.

Then the link to the short post on clodynights claimed it is too easy to discuss and already discussed enough. And when I read the CN post about ED I get the impression that ED need´s to be discussed from case to case.

"In many cases the term ED is nothing more than a marketing ploy." But not always?

So it means nothing then, too small part of the construction to be worth mentioning when we discuss pros and cons with different bins? The strap is more important? ;)

[Howard220]

One of the things I remember Roland Christen saying is that just because one element is "ED" doesn't mean the objective is any good. Among several factors that make it good, the optician needs to properly match it to the other glass element(s) that make up the rest of the objective. He is specifically talking about telescopes, though, not binoculars. So yes, the strap may be more important. :-)

[ronh]

This is may not be exactly in the spirit of this discussion, but I meant to post it anyway and here seems like a good place. It is just some very simple thoughts and activities, trying to understand color aberrations and how ED might help.

Not long ago, I described "Stokes's Test", which is masking off half of an objective lens. Devised in the days of long achromatic astronomical telescopes (some of us dinosaurs still use them!) it casts the color error of an achromatic objective into glaringly visible lateral color fringes. The test is equivalent to a misplacement of the eye behind the binocular, which occludes part of the exit pupil and which creates similar color effects.
Here's a link to that discussion, for anybody interested: Stokes's test (http://www.birdforum.net/showthread.php?t=126414&highlight=partially+explained)

This morning I did a couple of easy experiments with my Leica Trinovid BA, a normal glass binocular.

First I looked at a black telephone pole, with a bright sky in the background. Keeping my eyes centered behind the eyepieces, I moved the pole to the right side of the field, and yellow-green fringes appeared on the inner edge of the pole. Then, by moving my eye to the left, to recenter my eye on the now off-center exit pupil, I made the fringes disappear. By moving my eye even further to the left, occluding the other side of the exit pupil, I induced purple fringes on that same edge.

Second, with eyes centered, and left edge of pole centered, there were no fringes visible. But when I masked off the right side of an objective with my hand, purple fringes appeared, as predicted by Stokes for achromatic objectives.

I believe these tests demonstrate that much of the lateral color commonly seen in binoculars originates in the objectives, and is made visible by partial occlusion of the exit pupil due to eye placement. As part of the previous discussion, Henry Link tried Stokes's test with a Zeiss FL and got results quite different from what is expected with an achromat. I believe his result, and my results, taken with the frequently reported reduction in color aberrations in binoculars having ED glass objectives, supports the idea that ED objectives, at least when properly implemented as by Zeiss, are indeed the cat's pajamas, with regard not only to the subtle longitudinal, but also to the obvious lateral, color aberrations in binoculars.

I think as well that these tests largely exonerate the eyepieces from blame as the source of colored fringes.
Ron

[ThoLa]

This morning I did a couple of easy experiments with my Leica Trinovid BA, a normal glass binocular.
Ron

What does "normal glass" mean?

[Pileatus]

What does "normal glass" mean?
Tom,

That would be glass that isn't abnormal.

John

[ThoLa]

Tom,

That would be glass that isn't abnormal.

John

So how can a Trinovid be "normal" then?
In the Leica catalogue I found this:

""Optical glasses with anomalous partial dispersion Light rays of different colors are refracted at different angles through
the lens elements. This effect is called dispersion and in different types of glass it is present at different levels of strength. Most
types of glass have typical, “normal” charac-teristics. Special glasses with “anomalous partial dispersion”, on the other hand, have
characteristics that are different in certain ranges of colors, and this makes a special color error correction possible that cannot
be achieved with normal types of optical glass. Glasses with anomalous partial dispersion are used for the enhancement of image
quality, and they are used in all Leica binoculars and spotting scopes."


... and they are used in all Leica binoculars !

Funny, isnt it?


Glad not contain any glass (otherwise I'd start to wonder which category I might fall into),
T

[ronh]

Normal in the relationship between the dispersive, and refractive powers of the glass, as a function of wavelength.

Most glasses, like the crowns and flints used in achromatic lenses, lie along the same line on a plot of these parameters. Since the converging crown element must have more refractive power than the diverging flint element (or the lens would not converge light), the dispersive powers of the two elements, while also opposite in sign, are also unbalanced, thus secondary spectrum is only partially cancelled. "ED" glasses lie well off that line, and enable the cancellation of secondary spectrum when combined with the appropriate normal glass mating element.

I don't want to pretend to expertise here! I've done a little bit of reading, mostly with regard to telescope objectives. I am "pretty sure" the older Leicas use only normal glass, even though once there was an oblique reference to abnormal glasses in a vague brochure covering both Leica's camera and sport optics products.
Ron

[Kevin Purcell]

Normal glass in this case means a non-ED achromatic doublet crown glass a flint glass. Perhaps as Edz suggests an FK5 fluorite crown and SF1 flint or perhaps more commonly a BK7 or E-BK7 borosilicate crown with a flint.

Unless anyone can find where Leica documented it. I presume as they made the leap to HD that the weren't using flurophosphate glass in the older bins (otherwise they would have made a noise about it).

http://www.cloudynights.com/ubbthreads/showthreaded.php/Cat/0/Number/3193178/page/1/view/collapsed/sb/5/o/all/fpart/1

And from that thread this link which has some interesting comments (e.g. about FK5 and ED based doublets generally)

http://geogdata.csun.edu/~voltaire/roland/color.html
http://geogdata.csun.edu/~voltaire/roland/ed.html

Thanks for posting info on the Stokes Test , ronh, very interesting!

[ronh]

Tom,
I have read that in the product literature, and I thought the reference was sort of vague regarding the Trinovid, which shows very little in the way of especially good color performance, as far as I can tell. Yes, a literal reading clearly says the Trinovid contains abnormal glasses. But, I don't really believe it, and find such hazy misstatements are not uncommon in advertisements. Am I being too mean to not believe it, do you think? I'd hate to be considered slanderous, but it's better than over-defending one's binocular!
Ron

[kristoffer]

Tom,
I have read that in the product literature, and I thought the reference was sort of vague regarding the Trinovid, which shows very little in the way of especially good color performance, as far as I can tell. Yes, a literal reading clearly says the Trinovid contains abnormal glasses. But, I don't really believe it, and find such hazy misstatements are not uncommon in advertisements. Am I being too mean to not believe it, do you think? I'd hate to be considered slanderous, but it's better than over-defending one's binocular!
Ron

He has huge trust for the Leica catalogue, even when they are wrong, so be careful ;)

[FrankD]

Not to add more fuel to the fire but I have recollection of some form of extra low dispersion glass being utilized in past Leica products...particularly the Leica BN. If I remember the discussion correctly Leica stated that they had utilized it in their binoculars for some time but chose not to advertise it. That in and of itself sends up a red flag for me but I thought to comment on it regardless.

[Kevin Purcell]

If they were using a fluorite crown then one might call N-FK5 (i.e. a lead free FK5 glass) as "special" (i.e. not boring old BK7) as it is a slightly anomalous dispersion (i.e. the refractive index increases with increasing wavelength) but not say N-FK51 (a fluorphosphate crown ... an "ED" glass) that would be used with a special anomalous short flint (like N-KZFS4, N-KZFS5 or N-KZFS8).

http://refractiveindex.info/index.php?group=SCHOTT&material=N-FK5
http://en.wikipedia.org/wiki/Dispersion_(optics)

However there is at least on person, Roman Duplov, who believes that you can build apo telescopes without glass with anomalous dispersion. Even simple cheap BK7 and F2!

His paper was published in Applied Optics (peer reviewed so it's not completely bogus).

http://bossanova9.org/astro/optics/normal%20glass%20apo%20-%20duplov.pdf

Roman Duplov
Department of Physics, Donetsk State Univesity Ukraine.
roman@duplov.com

ABSTRACT: In order to correct secondary longitudinal chromatic aberration in conventional refracting optical systems, it is necessary to use at least one optical material having anomalous partial dispersion. A novel lens system with correction of the secondary spectrum by using only normal glasses is presented. The lens system comprises three widely separated lens components; both second and third components are subaperture. The presented example of an apochromatic telescope demonstrates secondary spectrum correction with the use of only crown BK7 and flint F2, which are among the most inexpensive optical glasses available at the market. Two more similar designs are presented, both with the use of low-cost slightly anomalous dispersion glasses. These telescopes have a higher relative aperture and a smaller tertiary spectrum.

And that low-cost slightly anomalous dispersion glasses is N-FK5 apparently "yields [] solid apochromatic color correction":

The next design example having an aperture of 90mm with an aperture ratio of f/7 is shown in Fig. 1(b) and the specifications are listed in Table 2.

It is a little more complex, it has a cemented triplet as the middle component and a cemented doublet as the rear one. This design employs an inexpensive and slightly anomalous optical glass, namely N-FK5.

The combinations of this crown glass with other normal glasses do not result in significant reduction of the secondary spectrum when used in conventional lens systems. However, in the presented optical system the use of N-FK5 yields the solid apochromatic color correction.

See the PDF for the details.

Perhaps this is what Leica did in those non-HD bins or perhaps it's what they're doing in their HD bins ... do they specify the glass they use?

So perhaps the world isn't as simple as it's sometimes put ;) I suspect this is what edz is hinting at in his comments about "semi-ED" doublet with FK5 and short flint SF1.

See also comments on this thread (silly marketing revealed!):

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/3155532/page/0/view/collapsed/sb/5/o/all/fpart/1

and this one

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/3192475/Main/3190323

There you go ... plenty to read!

[ronh]

Maybe my Trinovid has abnormal glass, but it's color fringe performance is apparently more like binos that do not make the "ED" claim than those that do. Under harsh light, I see fringes maybe not exactly at, but annoyingly near, the center of the field. I have not even had the pleasure of trying a Zeiss FL outdoors, but many have reported "NO fringes in the central half of the field" with such instruments. Also, the Trinovid's behavior under Stokes's test is that of a normal-glass achromat objective.

I hope this Trinovid issue hasn't overshadowed my simple tests and conclusions, which I hoped might be of some interest to the more general ED-glass discussion, which is much-needed. To repeat my point, I think I see a way that ED objectives can improve not only (subtle and nearly invisible) longitudinal, but also (obvious and frequently complained about) lateral, color errors. Would somebody kindly get on my case about that?

I apologize for my role in sidetracking things, and hope the ED discussion continues.
Ron

[Kevin Purcell]

I hope this Trinovid issue hasn't overshadowed my simple tests and conclusions, which I hoped might be of some interest to the more general ED-glass discussion, which is much-needed. To repeat my point, I think I see a way that ED objectives can improve not only (subtle and nearly invisible) longitudinal, but also (obvious and frequently complained about) lateral, color errors. Would somebody kindly get on my case about that?

Doesn't edz support this in his cloudy nights thread?

[ThoLa]

He has huge trust for the Leica catalogue, even when they are wrong, so be careful ;)

of course I have much more trust in the "experts" speaking here o:D
of course.

[henry link]

Ron,

I had forgotten about the Stokes test, which actually I've never done to the FLs. I'll have to try it.

Kevin,

I don't think we'll be seeing Roman Duplov's APO designs showing up in binoculars or general purpose telescopes. Look at the details. For instance, the actual physical length of the 100mm f4.5 is over 10 times the aperture and the well corrected image area at the focal plane is tiny, less than 0.5 degrees. These are limited purpose designs. The dog may have been taught to sing, but only one note.

For those hoping to educate themselves here I should warn you that even with my limited unprofessional knowledge of optics I can see that the links provided have a very nice mixture of the true and the wayward. The internet is like being in a classroom with ten "professors" and ten "experts" talking at once, some are brilliant, some are misinformed, some know less than you do. Good luck sorting it out

[kristoffer]

For those hoping to educate themselves here I should warn you that even with my limited unprofessional knowledge of optics I can see that the links provided have a very nice mixture of the true and the wayward. The internet is like being in a classroom with ten "professors" and ten "experts" talking at once, some are brilliant, some are misinformed, some know less than you do. Good luck sorting it out

So back at square one. ;)

[ronh]

Kevin,
Nope, EdZ disapproved of my raising the lateral color issue in his recent ED glass discussion at Cloudy Nights, on the grounds that lateral color is not an issue in stargazing, which is in fact true. The discussion there continues, as though longitudinal color was a big issue, and as if ED binos were APO scopes. Oh well.
Ron

[ronh]

Henry,
Of course our discussions are somewhat dirtball. But, where is a book that tells how ED glass is employed in binoculars, and with what result? It is manufacturing black art as far as I can tell. All we have is one another. Now get up to the podium and tell us what the heck is going on with this ED business!
Ron

[edz]

Kevin,
Nope, EdZ disapproved of my raising the lateral color issue in his recent ED glass discussion at Cloudy Nights, on the grounds that lateral color is not an issue in stargazing, which is in fact true. The discussion there continues, as though longitudinal color was a big issue, and as if ED binos were APO scopes. Oh well.
Ron


Hmm, well there's a few misconceptions. But well, OK
edz

[Kevin Purcell]

For those hoping to educate themselves here I should warn you that even with my limited unprofessional knowledge of optics I can see that the links provided have a very nice mixture of the true and the wayward. The internet is like being in a classroom with ten "professors" and ten "experts" talking at once, some are brilliant, some are misinformed, some know less than you do. Good luck sorting it out

You should help people sort it out, Henry. That's not a very helpful comment.

BTW, the post about the non-special APO was more to illustrate the point about Leica using "special" (possibly FK glass) but non-ED glass in their objectives (perhaps with special short flints) that can give very good performance.

So not all

I think perhaps this is easier to see the step in improvement in less expensive (i.e. Chinese EDs) bins than he incremental improvement is in more sophisticated designed bins that don't have an ED component.

It seems like non-ED doublets like FK5/SF10 (or with perhaps another more exotic short flint) can give better performance than older BK7/short flint doublets.

Of course if the bin makers actually told us what glass they used this might be a bit more transparent. If they explained their designs the same would be true.

[ThoLa]

Henry,
Of course our discussions are somewhat dirtball. But, where is a book that tells how ED glass is employed in binoculars, and with what result? It is manufacturing black art as far as I can tell. All we have is one another. Now get up to the podium and tell us what the heck is going on with this ED business!
Ron

"The Book" is here:
http://www.wiley-vch.de/publish/dt/books/bySubjectPH00/ISBN3-527-40320-5/?sID=lbe7bnl4nv180bpmoihjoke5q6


It will give you the "full picture".
There may be others from other publishing houses.
All you have to do is to find a library which has it. Or get a copy from your bookstore.

T

[ThoLa]

"The Book" is here:
http://www.wiley-vch.de/publish/dt/books/bySubjectPH00/ISBN3-527-40320-5/?sID=lbe7bnl4nv180bpmoihjoke5q6
T


and another one:
http://www.wiley-vch.de/publish/dt/books/bySubjectPH00/bySubSubjectPH40/3-527-40380-9/?sID=lbe7bnl4nv180bpmoihjoke5q6


You may also ask one of the experts here on BF; they will have these fundamental reference works in their private libraries.
Maybe you can borrow it for the weekend.
T

[elkcub]

You should help people sort it out, Henry. That's not a very helpful comment.

Henry's comments were "spot on," including his masterfully written post #2. As inconvenient as it may be, aberration correction is a very complicated subject and impossible to simplify in a few sentences — that is, even assuming one knows the subject in the first place.

A book that's readily available is Warren J. Smith's Modern Optical Engineering (1990). Chapter 3 on "Aberrations," and Chapter 7 on "Optical Materials and Interference Coatings," pg. 163-183, are particularly relevant to this discussion, and helpful for those who wish to sort it out. But the sorting will take effort, and I believe some familiarity with the underlying notions of calculus, even though the author takes great pains to avoid it.

Personally, I develop real heartburn trying to follow all the professors and "experts" Henry mentioned.

(Note: The book is available from ABE for less than $20 USD.)

[jcbouget]

This morning I did a couple of easy experiments with my Leica Trinovid BA, a normal glass binocular.

First I looked at a black telephone pole, with a bright sky in the background. Keeping my eyes centered behind the eyepieces, I moved the pole to the right side of the field, and yellow-green fringes appeared on the inner edge of the pole. Then, by moving my eye to the left, to recenter my eye on the now off-center exit pupil, I made the fringes disappear. By moving my eye even further to the left, occluding the other side of the exit pupil, I induced purple fringes on that same edge.

Second, with eyes centered, and left edge of pole centered, there were no fringes visible. But when I masked off the right side of an objective with my hand, purple fringes appeared, as predicted by Stokes for achromatic objectives.

I believe these tests demonstrate that much of the lateral color commonly seen in binoculars originates in the objectives, and is made visible by partial occlusion of the exit pupil due to eye placement. As part of the previous discussion, Henry Link tried Stokes's test with a Zeiss FL and got results quite different from what is expected with an achromat. I believe his result, and my results, taken with the frequently reported reduction in color aberrations in binoculars having ED glass objectives, supports the idea that ED objectives, at least when properly implemented as by Zeiss, are indeed the cat's pajamas, with regard not only to the subtle longitudinal, but also to the obvious lateral, color aberrations in binoculars.

I think as well that these tests largely exonerate the eyepieces from blame as the source of colored fringes.
Ron
Two days ago, I made a similar experiment with three binoculars on the full moon. The full moon makes the fringes more visible, and confirms the results I get in daytime.

The first binocular is the Fujinon 16x70 FMT SX2, that is known for its perceptible chromatic aberration in daytime.
When the lunar limb is centered, and the eye is perfectly centered, there is no trace of CA. But as soon as the eye moves to the right, there is a blue fringe that appears in the black sky at the left of the limb. When the eye is totally decentered, the blue fringe is very large and bright. The aspect of the fringe is very dependent on the eye position.

I agree this is a variation of the Stokes’ test.
The upper figure is an illustration of this situation. Here the eye is decentred to the top. Only the edge of the objective is used to form the image, and unfortunately, this is the worst part for chromatic aberration.

Then I put the lunar limb off axis. Again, I saw a large and bright fringe, very similar to the previous one. This situation is illustrated in the lower figure : in order to see the off-axis image, the eye must rotate in its orbit, in this causes exactly the same vignetting as previously. Again, the aspect of this fringe is very sensitive to eye placement.
In this case, what we call lateral CA seems to be due mainly to the objective.


The second binocular is the Zeiss 15x60 BGAT. When the lunar limb is on axis, moving the eyes creates a purple fringe. Compared to the Fujinon, the fringe is much fainter and narrower. Obviously, Zeiss could have been promoted extra-ED-super-APO objectives for these binoculars ;).

When the moon is off axis however, the fringe is totally different : there is a very narrow and intense red fringe, that seems to be less sensitive to eye placement. Maybe this is the true lateral CA due to the eyepiece, that is visible only because there is very few longitudinal CA.


The third binocular is the Nikon SE 12x50. On axis, the behaviour is very similar to the Fujinon 16x70 : a blue fringe sensitive to eye placement, but fainter and narrower than in the Fujinon. But compared to the purple fringe in the Zeiss, this blue fringe is more prominent. It’s quite an achievement for the Zeiss considering its higher magnification and larger objectives !

Off-axis, there is a narrow and intense red fringe, that seems almost identical to the Zeiss. Here also, I believe this could be the lateral CA due to the eyepiece.
The most interesting, is that moving the eye is this situation creates a blue fringe, wider and fainter than the red fringe, that reminds me of the blue fringe on axis. The red fringe remains visible near the limb.
My interpretation is that off axis, I can see in the same time the true lateral CA of the binocular as a rather constant red fringe, and the longitudinal CA as a variable blue fringe.


All these observations should be confirmed with my other binoculars and other experiments with a booster. I don’t consider they are definite conclusions. But basically, I agree with Ron’s comments, even if the subjet is complex.

Jean-Charles

[ThoLa]

A book that's readily available is ...


"Gross, Herbert et al. (2006):
Handbook of Optical Systems
Volume 3: Aberration Theory and Correction of Optical Systems
Optical Systems (vol 3)

756 pages, Hardcover
ISBN-10: 3-527-40379-5
ISBN-13: 978-3-527-40379-0

Written by industrial experts, this six-volume, full-color handbook provides a comprehensive introduction to the calculation, layout and understanding of optical systems, combining for the first time theoretical aspects of optical modeling with practical optical design.

Vol.3: Aberration Theory and Correction of Optical Systems:

29 Aberrations
30 Image Quality Criteria
31 Correction of Aberrations
32 Principles of Optimization
33 Optimization Process
34 Special Correction Features
35 Tolerancing "

[ronh]

Jean-Claude,
Thank you for doing the tests and reporting. I agree that the red fringes, which are characteristic of primary, rather than secondary color, do not arise in the objectives.

If all three of your binoculars were of the same design and quality, and the same objective focal ratio, I would expect the biggest, the 70mm, to be the worst, and the smallest, the 50mm, to be the best, as far as objective-induced color. That is the way achromats work. They need to get slower as they get bigger, but in binoculars they usually don't. The 60mm Zeiss beating the 70mm is not surprising, but beating the 50mm shows its very high quality.
Ron

Tom and Ed,
Indeed there is much to be learned. We really ought to knuckle down with some serious booking.how come none of these guys frequent our bino forum? (http://www.optics.rochester.edu/academic_programs/phd/phd_program.html)
Ron

[elkcub]

...
Tom and Ed,
Indeed there is much to be learned. We really ought to knuckle down with some serious booking.how come none of these guys frequent our bino forum? (http://www.optics.rochester.edu/academic_programs/phd/phd_program.html)
Ron

Probably too busy studying. ;)

[Hermann]

The second binocular is the Zeiss 15x60 BGAT. When the lunar limb is on axis, moving the eyes creates a purple fringe. Compared to the Fujinon, the fringe is much fainter and narrower. Obviously, Zeiss could have been promoted extra-ED-super-APO objectives for these binoculars ;).

As an aside: The Zeiss West 10x50 introduced in 1957 had what Zeiss called "semi-apochromatic" objective lenses. Somewhere in the old literature there's even some reference to the glasses they used, and the reduction of the secondary spectrum is quite visible when you compare it to other 10x50s made at the time.

Hermann

[ThoLa]

Tom and Ed,
.....how come none of these guys frequent our bino forum? (http://www.optics.rochester.edu/academic_programs/phd/phd_program.html)
Ron

how come none of these guys frequent our bino forum? o:)o:)o:):-O:-O:-O

splendid:t::t: