"Better Glass" (1 Viewer)

[Tringa45]

How many times have we read that on Birdforum?
However, none of us consumers has much idea what glass is used in our optics and what is "better glass" anyway?
Some manufacturers of astronomical telescopes advertise low dispersion glass such as FPL-53 used in their objectives, Kowa use Calcium fluoride (an artficially grown crystal, not glass) in their 55, 88 and 99 scopes and Zeiss used fluoride doped glass in the FL binoculars, but that's about the extent of information available to us.
Glass is basically Silicon dioxide (quartz) with the addition of other oxides, halogenides etc. to give it different refractive and dispersive properties. It can be considered as a supercooled liquid as it has no defined melting point, but the viscosity at ambient temperatures is so enormously high that there would be no measurable change in dimensions over decades.

Apart from transparency, the two most important properties of optical glasses are their refractive indices and Abbe numbers.
When light enters a dense medium (water, glass) from a less dense medium (vacuum, air) it is slowed and the refractive index of a glass is the ratio of the speed of light in vacuum to the speed of light in the glass. As the speed of light in vacuum is about 300.000 km/s, this would be rather difficult to measure!
One could imagine a wave front hitting a glass surface at an angle (angle of incidence is that to the perpendicular) and as it is slowed it is deflected towards the perpendicular (angle of refraction). The refractive index is the ratio of the sines of the angles of incidence and refraction.

However, refraction also depends on the frequency of the light and short wavelengths (blue) are refracted more than longer wavelenths (red), so if the optical designer has been less successful in correcting this, we see colour fringing (chromatic aberration) in our binoculars.
The Abbe number expresses the differences in the refactive indices of an optical glass at both ends of the spectrum with respect to the refractive index in the middle of the visible spectrum. The higher the number, the lower the dispersion.
Crown glasses generally have a relatively low refractive index and low dispersion (high Abbe no.) and flint glasses a high refractive index and high dispersion (low Abbe no.).
A crown glass example is Schott BK-7 (Ref. 517642), which indicates a refractive index of 1,517 and an Abbe no. of 64,2.

By combining optical glasses it is possible to attain refraction while minimizing dispersion and designers have a wide choice available with more than 120 varieties available from Schott alone, some in high transmission versions and others with extremely tight tolerances in refractive index and a minimum amount of bubbles in the glass blanks.
By combining a positive crown element and a negative flint element achromatic correction (2 colour wavelengths) is possible but apochromatic (3 wavelengths) or super-apochromatic correction (4 wavelengths) usually demands complex designs with exotic (and expensive) glass formulations.

For those interested, here is a useful database Refractive index of CH4 (Methane) - Rollefson, which includes not only Schott, but other optical glass manufacturers.

Corrections or additions to the above welcome, as always.

John

 

[Mono]

I do also think the phrase "better glass" might reflect the American parlance of referring to binoculars as "glass".

 

[Tringa45]

I do also think the phrase "better glass" might reflect the American parlance of referring to binoculars as "glass".

No, the references were often to the material ("better coatings, better glass"), not the instrument. Hence the thread.

 

[Mike Crawley]

For me, this raises a number of questions
How much further development is possible for optical glass (silicon dioxide) ?
Or is are the current materials used by Swarovski, Zeiss, et al already developed as far as possible ?
Or is it becoming more about coatings ?
Or is it advancement through novel materials (i.e. calcium fluoride or other experimental fluoride containing compounds) ?

and I assumed binoculars when I saw “glass” in the thread title

 

[Maljunulo]

Use of the term “optical glass” may clarify, since “glass” seems to be ambiguous.

 

[Tringa45]

For me, this raises a number of questions
How much further development is possible for optical glass (silicon dioxide) ?
Or is are the current materials used by Swarovski, Zeiss, et al already developed as far as possible ?
Or is it becoming more about coatings ?
Or is it advancement through novel materials (i.e. calcium fluoride or other experimental fluoride containing compounds) ?

and I assumed binoculars when I saw “glass” in the thread title

As a layman I have no idea. Where should one push the boundaries: https://www.schott.com/en-gb/produc..._en1.jpg?rev=1bf1a4b8ca874639b18f1a0aa8c42655 ?
Seriously though, Ohara have pushed the Abbe no. up to around 95 with FPL53 and FPL55, similar to CaF2. They might like all optical glasses show some diffusion compared to fluorite and are similarly, I believe, rather expensive and fragile.
AFAIK, broadband coatings now have up to 7 layers and can transmit >99,8% per surface.

John

 

[Scott98]

In amateur astronomy we talk about how the lens functions as a whole being more important than the type of glass within. The dispersion qualities, transparency, etc., of the individual glasses in an optical instrument do affect the overall quality but it's only one factor of many.

 

[William Lewis]

I had a bit of fun with this recently looking to see if I could modernise the glass in my zeiss decarem with modern coatings but hit the minefield you've mentioned above.

I was looking at the various different glasses available that local to me (Cambridge based) companies could form to the correct shapes i.e the same as the original glass to work in the decarem. It soon became apparent from my research that changing one glass element for a different type of glass would mean changing not just the glass type but also potentially the shape of the other elements. I decided it was too much of a minefield so just sent them in for a good service!

It does raise an interesting question though, when we talk about binoculars with a traditional optical design, classic example being the habicht 7x42, is the optical design actually the same or has each element had to be changed in shape to make the most of the modern glass and coatings?

Will

 

[Mike Crawley]

As a layman I have no idea.

That’s OK, I wasn’t expecting an answer.
Just a few of the things which occur to me when considering potential/possible advancements in our hobby

 

[Tringa45]

It does raise an interesting question though, when we talk about binoculars with a traditional optical design, classic example being the habicht 7x42, is the optical design actually the same or has each element had to be changed in shape to make the most of the moder glass and coatings?

Will,
The Habichts have fairly simple doublet objectives so if any changes were made, to replace leaded glass for instance, I think they would have been fairly small.
I did read however, that Roland Christen of Astro Physics made changes in lens curvature to compensate for minor variations in refractive index of different batches of the same glass specification.

John

 

[Maljunulo]

It’s like a spiderweb, where if you pull on one strand, the whole web responds.

Everything interacts with everything else in the optical train. Changing the physical properties of a single element affects the whole train.

Change the curvature of one surface, or alter the properties of the glass in a single element, and back to the beginning.

 

[Tringa45]

One of the biggest differences between inexpensive binoculars and more expensive binoculars is the quality of the glass and the coatings, and it makes a big difference in the quality of the view, and as an amateur birder that is all you really have to know.

Glass Quality | Best Binocular Reviews

Many people ask me to explain what the difference is between high end binoculars that can cost well over $1000 (£1000) and then at the other end of the spectrum cheap binoculars costing less than $100. The answer is that there are many differences, not just one. From build quality to the quality...

www.bestbinocularsreviews.com

That is precisely the sort of naive speculation I was trying to debunk.
I don't know, you don't know, and I seriously doubt that the reviewer is privy to Leissovski's secrets or able to understand the complexities of lens design.
One of the last complex camera lenses designed without the aid of computers was the Zeiss Biogon for the Hasselblad Super Wide camera.
The calculations allegedly took several years!

John

 

[tenex]

I'm not quite sure what the intended context of "better" was here. If it's comparison between optics with more and less sophisticated glass, that's really too difficult for lack of information from manufacturers who may mention only BaK4 or FL, or worse, "HD". If it's further potential progress, somehow that seems unlikely in binoculars at least, given the wide variety of properties already available. Or one could turn the question around: what improvements could still be hoped for in binoculars through the use of more exotic glass? Recent gains seem to arise mainly from coatings. (Camera lens makers use many types of exotic glass now such as anomalous partial dispersion, and some publish cutaways showing where... yet still rely increasingly on software correction of images.)

 

[John A Roberts]

Hi William (post #8),

One change that we do know about on the Habichts, is that the eyepiece shared by the 7x42 and 6x30 versions was changed around 1965.

The same number of lenses and grouping were used, but with significant alterations:
• the diameter, shape and thickness of the lenses in the rear group were increased;
• the shape and thickness of the front lens were changed, and;
• the spacing between the groups was decreased.

It’s one change that we’re aware of because of the externally visible change to the eye lens.
See the images in post #128 at: Why are those dang Habicht's so BRIGHT!
And a cross-section of the current eyepiece, is way back in post #10 in the same thread, shown on the SL 7x42.


More generally, as with many items during production, optical manufacturers might 'tweak' the glass
(and other components) for a variety of reasons, including:
• changes to the availability of materials, such as reliability of sourcing, consistency of product and price, and;
• engineering fixes, where there’s an easier/ better/ cheaper way to achieve the same outcome.

It’s just that most of the time we’re unaware of such things.


John

 

[rodneyAB]

That is precisely the sort of naive speculation I was trying to debunk.
I don't know, you don't know, and I seriously doubt that the reviewer is privy to Leissovski's secrets or able to understand the complexities of lens design.
One of the last complex camera lenses designed without the aid of computers was the Zeiss Biogon for the Hasselblad Super Wide camera.
The calculations allegedly took several years!

John

That 38mm Biogon SWC holds it's own in the modern world. occasionally use a 1969 SWC, always a treat.

 

[has530]

Other factors to consider along with optical design and glass types are manufacturing tolerances. The amount of bubbles allowed in the glass, surface roughness of lenses, lens shape tolerance etc...

 

[Patudo]

if I could modernise the glass in my zeiss decarem with modern coatings

if yours are already multi-coated they'll be OK; Zeiss Jena's T3M coating wasn't as good as Zeiss West's T* but is just about good enough (for me anyway, and for their price).

I wonder how interchangeable the optical elements of the Dekarems and other binoculars that evolved from single to multi-coated (or from partly to fully multi-coated, like Swift Audubons and some Nikons) were. For instance - can you slot in multi-coated eyepiece elements (for instance) interchangably between single and multi-coated Dekarems?

Ironically it seems coating lenses is easier if they are completely uncoated. I wonder how the type of multi-coatings used on spectacles/glasses would work on say an old uncoated Zeiss Silvarem or similar?

 

[bsantini1]

Will,
The Habichts have fairly simple doublet objectives so if any changes were made, to replace leaded glass for instance, I think they would have been fairly small.
I did read however, that Roland Christen of Astro Physics made changes in lens curvature to compensate for minor variations in refractive index of different batches of the same glass specification.

John

When seeking perfirmance at the absolute optical limits definedby physics, you HAVE to individually compensate for melt data. There is no choice.

B

 

[Scott98]

you can't remove coatings without grinding the glass off and re-figuring and polishing, re-coating is not going to happen w/ binoculars

 

[Binastro]

I doubt whether many binocular makers correct for individual melts.

They probably check that it is within normal limits.

The only binoculars I suspect might adjust curves and thickness are the Zeiss 20x60S, Nikon WX and Takahashi 22x60.

Some binocular telescopes probably.

Normal binoculars are so poor optically, I doubt it is cost effective.

Professional lenses and military lens makers may make adjustments, but maybe by spacing changes.

Chinese glass blanks have been routinely rejected because they just are not up to specs.
They are used because they are cheap to buy and allow firms to be competitive in pricing.

Regards,
B.