Plastic lenses (1 Viewer)

[Curiouswind]

At the risk of again offending more experienced and knowledgeable members, I have a couple of questions. Yes, I have tried to search the internet for answers.

Years ago, my eyeglasses had glass lenses. My optician convinced me to switch to plastic lenses, trivex in my case. He said these lenses provided equal vision correction, superior impact resistance, lighter weight and, possibly, lower cost. My lenses have a scratch-resistant coating that was guaranteed for one year even if I used my shirt tail to clean the lenses.

My first question is why don't binoculars have a scratch-resistant coating over the existing ones?

Secondly, what advantages do glass lenses have in binoculars that would be lacking in eyeglass applications?

Thanks for any answers that do not mention bowling balls.

 

[tenex]

With proper care, glass shouldn't need scratch-resistant coating, because it's harder. And glass comes in many varieties with different optical properties that are useful in various combinations (typically 9-10 elements in each tube) for controlling aberrations.

Incidentally, my plastic-lensed sunglasses get much more smeary and are harder to wipe clean than old glass ones, or binoculars. So a question of my own: is that static electricity, or what?

 

[jring]

Hi,

the usual MgF multicoatings are very hard as compared to plastic, even when the plastic is coated. Wiping a lens with MgF coating with a shirt tail is not a problem. It gets problematic when the lens or shirt tail is contaminated with mineralic dust or sand - that will result into scratches unless your lenses are made from diamond...

But the main problem with plastic lenses is, that we don't have the technology to grind them to the specs needed for high quality optics - eyeglasses don't need to apply - their tolerances are way larger than what is needed for binoculars, let alone telescopes. The plastic lenses are being manufactured by pressing the plastic into a mold - not that precise but you one can easily make aspheric surfaces, which helps the optics designer but they are quite hard to make in glass - often done by hand-finishing by a master optician...

Joachim

 

[Tringa45]

My first question is why don't binoculars have a scratch-resistant coating over the existing ones?

Swarodur and Meoshield for example.

Secondly, what advantages do glass lenses have in binoculars that would be lacking in eyeglass applications?

AFAIK the highest Abbe no. among plastic lenses is 59 for CR-39. That is about equivalent to BK-7, the old established crown glass. Most othe plastic materials are in the region of flint glass, so colour correction in binoculars or photographic lenses requiring much higher refractive properties than eyeglasses would be impossible to achieve. Nothing equivalent to FPL-55 or fluorite in plastic.

John

 

[Curiouswind]

Hi,

the usual MgF multicoatings are very hard as compared to plastic, even when the plastic is coated. Wiping a lens with MgF coating with a shirt tail is not a problem. It gets problematic when the lens or shirt tail is contaminated with mineralic dust or sand - that will result into scratches unless your lenses are made from diamond...

But the main problem with plastic lenses is, that we don't have the technology to grind them to the specs needed for high quality optics (eyeglasses don't need to apply - their tolerances are way larger than what is needed for binoculars, let alone telescopes. The plastic lenses are being manufactured by pressing the plastic into a mold - not that precise but you one can easily make aspheric surfaces, which helps the optics designer but they are quite hard to make in glass - often done by hand-finishing by a master optician...

Joachim

Thank you. I did not realize that eyeglass lenses are not as precisely made as those in binoculars.

 

[Curiouswind]

Swarodur and Meoshield for example.

AFAIK the highest Abbe no. among plastic lenses is 59 for CR-39. That is about equivalent to BK-7, the old established crown glass. Most othe plastic materials are in the region of flint glass, so colour correction in binoculars or photographic lenses requiring much higher refractive properties than eyeglasses would be impossible to achieve. Nothing equivalent to FPL-55 or fluorite in plastic.

John

Thanks. Again, I had not realized the differences between eyeglass lenses and binocular lenses.

 

[Curiouswind]

With proper care, glass shouldn't need scratch-resistant coating, because it's harder. And glass comes in many varieties with different optical properties that are useful in various combinations (typically 9-10 elements in each tube) for controlling aberrations.

Incidentally, my plastic-lensed sunglasses get much more smeary and are harder to wipe clean than old glass ones, or binoculars. So a question of my own: is that static electricity, or what?

I always managed to scratch my glass lenses, mostly while cleaning them. My current plastic lenses have not scratched despite being cleaned just the same way. The optician's claims are correct and I haven't had to take him up on the guarantee.

I've always assumed that the coatings (AR, anti-scratch) are the reason that my plastic lenses get dirtier faster and are harder to clean than the glass ones. I'll have to ask my optician the next time I see him.

 

[Binastro]

There are probably 100 million camera lenses made from plastic from the 1950s onwards.
These were on low end cameras.

I think that smartphone camera lenses may also be plastic, so that means billions of plastic lenses.

I had a large compound lens from a photocopier. It had garish strange coatings quite unlike those found on glass lenses.
It was plastic maybe by Tamron.

There was a very interesting plastic Vivitar Series1 450mm f/4.5 mirror lens.
Although it went into production there were problems with keeping the high quality needed and it may have only been available for a year or two. I have never seen one.

Plastic/glass hybrid elements are found in high end photo lenses, but internally.

The Nikon Action binoculars have, I think, plastic aspheric elements in the eyepiece.

There is not the range of optical plastics to compare with glass, and would probably not be used in external elements.

I think that some polishing may be done with molded plastic elements.

Toy binoculars and telescopes may have plastic optics.

A binocular using plastic elements might just be longer, more fragile and get scratched externally.

Glass is not that great always. Early Leica camera lenses were notoriously fragile, in that the front element needed refiguring and polishing as a soft unreliable glass type was used.
High end glass elements tarnish immediately and have to be multicoated as soon as they are made.
Some also devitrify and absorb moisture.

Regards,
B.

 

[fazalmajid]

I have fairly expensive Seiko Brilliance varifocals and I'd like to keep them pristine, so the way I clean them is with soap under running water, then I blow away the water drops with an air compressor. No cloths to potentially cause scratches with trapped grit.

 

[abirdingguy]

As dumb as this question sounds, will you please explain a bit more about your technique for cleaning eyeglasses. I am guilty of sometimes using cleaning clothes, even ones that have been previously used.

Is this also your method of cleaning (glass) binoculars and scopes? I am more careful with them since they are more expensive and aren't replaced every year or two like eyeglass lenses. There is probably room for improvement here too though.

 

[Alexis Powell]

...my plastic-lensed sunglasses get much more smeary and are harder to wipe clean than old glass ones, or binoculars. So a question of my own: is that static electricity, or what?

Plain glass wipes clean fairly easily, plastic does not, but if you buy plastic glasses, including sunglasses, that include hydro and oleophobic coatings, you will find them easy to wipe clean (though best practice is to use a bit of detergent and water). Lenses, whether glass or plastic, with antireflective coatings but lacking hydro and oleophobic coatings are particularly hard to clean, and not just because the coatings make the oil smudges easy to see. I think oil adheres to antireflective coatings better than it does to glass.

--AP