However now that I think about it -- even that question is not clearly answered when thinking about it -- "The doublet is made from glass containing calcium fluorite"
It doesn't explicitely state that both elements contain it.
And btw -- I guess the typo was made by moderator Troubador aka Lee who typed down the recorded interview if I understood the post correctly.
Also -- "Flourit" and "Flourid" is pronounced about the same in German. The interview might have been in English though so I cannot say how the typo came about.
Thank you for spotting this error. The interview text was checked by two people before posting but this still got through. My mistake and with apologies I have now corrected this.
As to whether both elements contain fluorides, I am checking this with my source and will report back. However see the Zeiss-sourced diagram below and note that the German language concerning the objective group (which includes the focusing lens) states Ultra FL Linsen. The latter word, 'linsen' means lenses i.e. in the plural, so more than one lens contain fluorides. Also note that in the book published by Zeiss, Victory SF: The Art of Precision, it specifically states that the focusing lens is a fluoride lens.
Lee
Just to recap:-
Calcium fluoride (CaF2) or fluorite is a homogeneous crystal. It occurs naturally but is grown artificially for use in lenses by Canon Optron.
It has very low colour dispersion with an Abbe number of 95 and zero scatter, i.e. if one shines a laser through a fluorite lens, the laser path is not visible as it would be in glass.
Glass is an amorphous mixture of various substances and so-called fluoride glass is doped with metal fluorides, not necessarily calcium fluoride. It could be aluminium fluoride or beryllium fluoride, we just don't know and I don't think Schott or Ohara or Hikari etc. are going to give us their recipes .
I think it's quite likely that the fluoride glass used in the Zeiss SF is Schott FK51.
Magnesium fluoride, btw, is the substance used for single layer (1/4 wavelength) anti-reflection coatings. It has an Abbe number of 106 but I don't know if it would lend itself to lens fabrication.
John
Just to recap:-
Calcium fluoride (CaF2) or fluorite is a homogeneous crystal. It occurs naturally but is grown artificially for use in lenses by Canon Optron.
It has very low colour dispersion with an Abbe number of 95 and zero scatter, i.e. if one shines a laser through a fluorite lens, the laser path is not visible as it would be in glass.
Glass is an amorphous mixture of various substances and so-called fluoride glass is doped with metal fluorides, not necessarily calcium fluoride. It could be aluminium fluoride or beryllium fluoride, we just don't know and I don't think Schott or Ohara or Hikari etc. are going to give us their recipes .
I think it's quite likely that the fluoride glass used in the Zeiss SF is Schott FK51.
Magnesium fluoride, btw, is the substance used for single layer (1/4 wavelength) anti-reflection coatings. It has an Abbe number of 106 but I don't know if it would lend itself to lens fabrication.
John
Thank you for this John, I have amended post 21 accordingly.
Lee
A couple of German bino dealers' websites state SF has two fluoride lenses in the objective group so it is beginning to look like the focusing lens and one other element.
Lee
It's not hard to figure out which lens doesn't use fluoride glass. It's the one up front that's obviously thinner in the middle than at the edges (see post #21). In an objective group that means it's fulfilling the role of a "flint", which is never made of low dispersion glass.
Putting that element on the outside also seals off the internal fluoride containing objective elements from whatever environmental conditions might harm them.
I am not a chemist, but I question whether the fluorite element in say a Kowa spotting scope is made from one grown crystal.
My understanding is that it is fused calcium fluorite, which may contain striations.
This may be why the Kowa 99 seems to have problems in achieving the quality that one expects from an expensive scope.
Large sizes may be difficult to make well.
Fluorite crystal has problems with temperature.
It is difficult to work with.
It may deteriorate.
There may be more failures than with glass elements.
The early Canon 300mm f/2.8 lenses deteriorated so fast they became unusable.
I think that the coatings probably allow such lens elements to be commercially viable.
The coatings are probably more difficult to apply than to glass elements.
I wonder how wise it is to buy secondhand fluorite optics?
Regards,
B.
I am not a chemist, but I question whether the fluorite element in say a Kowa spotting scope is made from one grown crystal.
My understanding is that it is fused calcium fluorite, which may contain striations.
This may be why the Kowa 99 seems to have problems in achieving the quality that one expects from an expensive scope.
Large sizes may be difficult to make well.
Fluorite crystal has problems with temperature.
It is difficult to work with.
It may deteriorate.
There may be more failures than with glass elements.
The early Canon 300mm f/2.8 lenses deteriorated so fast they became unusable.
I think that the coatings probably allow such lens elements to be commercially viable.
The coatings are probably more difficult to apply than to glass elements.
I wonder how wise it is to buy secondhand fluorite optics?
Regards,
B.
I know some Takahashi telescopes also had issues with the Fluorite element. From what I recall they were older models and had the crystal as the front objective element. Later designs moved them to the second element to provide some shielding. I believe they have also gotten much better at coating it.
Regards to blank size. Canon has been putting the Fluorite further back in the lens on their most modern designs. The blank isn't full objective size. I assumed this was due to wanting to get a lighter lens and new technology allows such designs. Lens blanks of Fluorite or ED glass are harder to make in larger and larger sizes, but there are still telescope makers buying larger blanks than anything you would see in a spotting scope. Considering most of those are going to be used for astrophotography, the bar for quality is way higher than anything we would see in a spotting scope.
Actually, astrophotography for pretty pictures of deep sky objects probably doesn't need as high quality tolerances as for planetary visual high magnification use.
The corrections needed for astrophotography are flat field and good star images into the corners.
Also the use at full aperture, but this is often only 60mm.
But the need for magnifications of 200x and higher is not required.
Although for planetary astrophotography high quality is needed, and very good collimation.
Here the Celestron 14 is often the scope of choice, not a fluorite or ED scope.
The situation with ED and fluorite is much the same as Cooke's 11 inch photovisual telescopes, which were a big step up from traditional telescopes.
It was found that every seven years the objective element needed repolishing.
Until the telescope became unusable.
Regards,
B.
Why are Lens Coatings Used?
If you are looking for more details, kindly visit CLZ.
Imagine walking through a thick forest. Sunlight struggles to penetrate the dense foliage, making it difficult to see. Similarly, light encountering and then trying to pass through a series of lenses in binoculars encounters obstacles. Here’s what happens:
- Reflection: When light strikes an air-to-glass surface, a portion of it is reflected, just like light bouncing off a mirror. So because this reflected light doesn’t reach your eyes, it does not contribute to the image you see, effectively reducing the potential of the overall quality and brightness of the image.
- Scatter: Light can also scatter within the glass itself, causing the image to be less sharp, so more hazy and blurry.
Special lens coatings are applied to help combat these issues and improve the overall viewing experience through binoculars.
So with binoculars, scopes, monoculars and many other optical devices like camera lenses, anti-reflection coatings are designed to assist with light transmission thus helping to produce a sharper, brighter image with improved contrast. These days almost all good quality binoculars have some sort of anti-reflection coatings applied to at least some of the air-to-glass surfaces of their lenses.
How Do Lens Coatings Work?
These anti-reflection coatings are often made up of a number of incredibly thin layers, typically measured in microns (one-thousandth of a millimeter), that are deposited on the lens surface. Each has a distinct index of refraction which alters the lens’s performance with different wavelengths and at different incident angles and work by manipulating light in specific ways:
So they kind of function like a tiny “traffic cop” for light. The coating is composed of alternating layers of materials with different refractive indices (how they bend light). These layers cause incoming light to interfere with the reflected light, cancelling it out and allowing more light to pass through to your eye. Imagine two sound waves perfectly out of phase, cancelling each other out – that’s the principle at work here!
Because of this, color effects often appear at oblique angles and so you can sometimes see these coatings as they are usually what produces the blue, red, or green reflections you see when you look into the front of a binocular’s objective lens at different angles.
Improved Transmittance and Image Brightness
In any lens, some of the light that passes through the lens is reflected by the front (incident light) and rear (exiting light) surfaces. This reflection reduces the amount of light passing right through the lens and thus to your eyes.
If the amount of light reflected away is very bad, the image you see when you look through the binoculars will be darker than on binoculars that transmit more light.
As well as producing a less bright image, reflected light may cause ghosting, flares and also affect the contrast of the image.
So during the manufacturing process, these special anti-reflective coatings are added to the lenses on better quality binoculars, which drastically decreases the amount of surface reflection losses and can significantly increase the transmittance of light to ensure you get as bright, clear and sharp image as possible.
Ghost Imaging
What is more, the undesirable secondary reflections that can interfere with the transmitted image, producing what is known as “ghost imaging” can also be all but completely be eradicated using good anti-reflection coatings.
More Benefits:
Because the amount of reflection from the lenses is reduced, these coatings also help in that they reduce the glint coming off the lenses. This is important if the user of the binoculars does not want to give away their position. Important for military, law enforcement, hunting and even general wildlife observation applications.
Coated vs Multi-Coated vs Fully Multi-Coated
When researching a binocular, it is important to note how the manufacturer describes their coatings:
Coated – means a single layer antireflection coating on some lens elements, usually the first and last elements (the only ones you can see).
Contact us to discuss your requirements of Single layer magnesium fluoride coating. Our experienced sales team can help you identify the options that best suit your needs.
Fully Coated – means that all air-to-glass surfaces are coated, which is obviously an improvement on a single-layer coating.
Multi-coated – means that at least some surfaces (again, usually the first and the last) have multiple layers of antireflection coatings. (A multilayer coating effectively reduces reflected light that cannot be eliminated with a single-layer coating, and increases the transmittance of light.) Multiple layers are about an order of magnitude more effective than a single layer.
Fully Multi-Coated – means that all air-to-glass surfaces have received multiple layers of anti-reflection coatings – this is what you want in your binoculars.
The table below shows Transmittance by type of coating:
Per Single Lens Surface
10 Lens & Prism Surfaces
No Coating:
96%
(0.96) x Power of 10 = 0.66
66%
Single-Layer:
98.5%
(0.985) x Power of 10 = 0.86
86%
Multilayer Coating:
99.5%
(0.995) x Power of 10 = 0.95
95%
Materials Used in Lens Coatings
The specific materials used in lens coatings are a closely guarded secret by manufacturers. However, common materials include:
- Magnesium fluoride (MgF2): A popular choice for single-layer anti-reflective coatings due to its effectiveness and ease of application.
- Silicon dioxide (SiO2): Used in multi-layer coatings for its excellent light transmission properties.
- Tantalum pentoxide (Ta2O5): Known for its high refractive index, useful for creating the desired optical effects in multi-layer coatings.
- Aluminum Oxide (Al2O3): Chosen for their specific refractive indices and transparency to visible light.
Application of Lens Coatings
Applying lens coatings is a complex and precise process, often involving multiple steps:
- Surface Cleaning: The lenses are meticulously cleaned to remove any dust, oil, or contaminants that could affect the coating adhesion.
- Vacuum Deposition: The lenses are placed in a vacuum chamber where the coating materials (often in the form of pellets) are vaporized and deposited onto the lens surface in a controlled manner. Different techniques like sputtering or evaporation can be used for this purpose. For more on this process, take a look at this article and video on How Binoculars are Made.
- Monitoring and Quality Control: The thickness and uniformity of the coating are precisely monitored throughout the process to ensure optimal performance.
Cost of Fully Multi-Coated Binoculars
There was a time when you would only find fully multi-coated binoculars at the top end of the market. But as production methods improved and because of the influence of the cheaper production costs from mostly Asian manufacturers, more and more binoculars were being fully multi-coated.
So because of this increased competition and better manufacturing techniques, most mid-range and even some low-cost binoculars come fully multi-coated with anti-reflection coatings. So now a bin needs far more optical features, like for example the use of ED Glass in their lenses, to even have a chance of standing out from the crowd.
Best Value Fully Multi-Coated Bins
So to help with this and to help separate the wheat from the chaff, below I have listed a few of what I consider to be some of the best value for money fully-multi-coated binoculars around:
Low Cost
Mid Priced
Mid-High
High End
$50 – $130 /
£50 – £130
$130 – $300 /
£130 – £300
$300 – $500 /
£300 – £500
$500-$ /
£500-£
Pentax Papilio II 8.5x21
Hawke Frontier HD X 8x42
GPO Passion ED 8x42 Binoculars
Maven B1.2 10x42 Binoculars
More: View all Best Value Binoculars I have reviewed.
Additional Features of Lens Coatings
Beyond the core functionalities mentioned above, some manufacturers offer additional features through their lens coatings:
- Scratch Resistance: Certain coatings can add a layer of protection against scratches and abrasions, extending the life of your binoculars.
- Hydrophobic Coatings: These repel water droplets, preventing them from beading up on the lens and obstructing your view, particularly useful in wet conditions.
- UV Protection: Coatings can be designed to block harmful ultraviolet (UV) rays, protecting your eyes during extended use.
A Note on “Ruby Coatings”
There are some binoculars that advertise that they have ruby or red multi-coatings. These are intended to reduce glare in bright light, or enhance specific colors in specific environments and has had some limited success in very specific applications.
But due to the fact that it “looked cool”, many low-end binocular producers started making ruby-coated binoculars to filter red to compensate for their poor-quality optics that do not properly converge the color spectrum. For more: Ruby Coated Binoculars.
Conclusion
Anti-reflection lens coatings on binoculars play a crucial role in enhancing your viewing experience and are important to take note of when comparing instruments. By understanding how they work and the features they offer, you can make a better decision when choosing the right binoculars for your needs.
Remember:
For more Spherical Lensesinformation, please contact us. We will provide professional answers.
- Look for binoculars with fully multi-coated lenses for optimal light transmission and image quality.
- Consider additional features like scratch resistance, hydrophobic coatings, and UV protection depending on your intended use.
Further Reading:
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