(Theoretical) Optics Question

JK1231

Okay, I know this is somewhat off topic -- but where better to ask an optics question than on a photography forum?

Question: Why are lenses made of glass always sharper than lenses made of other substances, such as various optical plastics? Is there a substance other than glass that could make a sharper lens than a glass lens?

I have a very personal reason for asking -- my glasses. I've had glass lenses "forever" and my glasses weigh a ton. With my current prescription, my optician has been unable to find a lab willing to make it in glass. The lab that made my last three or four prescriptions says they've tried to make the prescription in glass several times and the lenses keep breaking when mounted.

I'm currently trying out high-index plastic lenses (ones with an index of refraction very close to glass), and I very much like the weight of the glasses, but the focus is very soft and beyond about a hundred feet, focus degrades rapidly. I've tried plastic lenses before, and with every previous try, I've also had an issue with sharpness and a loss of focus at distance which does not occur with glass lenses of the same prescription.

So, back to my questions:
1) What is it about plastic lenses that make them less sharp than glass? I've found the closer to glass the index of refraction is on plastic lenses, the sharper they are -- but why is that?
2) Why the loss of depth of field in plastic lenses relative to glass lenses?
3) Is there any substances that give a sharper focus than glass? If so, what about that substance makes the focus sharper?

Thanks in advance for any technical explanations anyone can provide.

I know this is somewhat off topic, but I'm certain that the same theoretical questions about vision glasses also apply to camera lenses! :-)

Thanks!

JK

sevencrossing

This is a guess. I suspect plastic lenses are likely to bend and therefore change their optical characteristics

Ironheart

I wear polycarbonate lenses, sharp as a tack, and bulletproof. The substance the light goes through, as long as it is perfectly clear, won't matter in terms of focus. In a modern camera lens, several of the elements may be something "other than glass" meaning fluorite, or other elements are added.

If your glasses aren't sharp, they are made incorrectly. Have your optician try a different lab and just use the most modern and most expensive lens material they have. That's what I just did for my newest pair, and I have better than 20/20 with my glasses, they darken automatically in the sun, scratchproof, smudge proof, lightweight, etc... I think you got a bum pair or you went cheap and actually got plastic rather than a more modern material.

spraynpray

Just thinking outside the box here - how do you know that plastic glasses are less good from a hundred feet? If the answer is that you've had the prescription made in plastic and tried them, then I suggest you use the plastic ones for a couple of weeks then try going back to the glass ones. You may then find the glass ones aren't sharp because your eyes have 'settled in' to the new plastic glasses. This is often the same with replacement glass glasses - they aren't quite the same from old to new.

@Ironheart: Interesting: Do you notice any CA with your new polycarbonate spectacles? Also, when you say 'scratch proof', it is the coating on glasses that usually gives up, are your coated and if so, the coating is usually softer than the base material and so will scratch.

manhattanboy

Is there a substance other than glass that could make a sharper lens than a glass lens?

Yes, see the silicon hydrogels used in contact lenses.

JK1231

Okay, a little more detail...

In the past 4 or 5 iterations of lens prescriptions, I've usually started with the "latest and greatest" high quality non-glass lenses. I've then worn those glasses for a couple weeks, had problems from day one with image softness and focus loss at distance, then had the lenses remade in glass (wearing the "plastic" ones while the glass ones were made -- usually 3 to 4 more weeks), then switched to the glass lenses when I received them -- and they were sharp and I had no problems at distance, ever. So, I don't have a problem with my eyes needing to adjust to my new prescription.

I've had both the optician and my ophthalmologist (no business relationship between them) check the non-glass lenses and the glasses have been made correctly, so that is not an issue.

The optician claims that all non-glass lenses are never as sharp as glass, and that it's his understanding that it is due to the difference in the index of refraction of the material. I will say that it has been my experience that the higher index plastics (ones closest to glass) are the sharpest and have the greatest depth of field.

So, I guess my real question is: How does the index of refraction of a material effect the sharpness of its focus and its depth of field?

JK

heartyfisher

i think the answer is its bec the plastic lenses need to be thicker to bend the light as much due to the lower index (density) hence the 2 surfaces are farther apart resulting in greater chromatic aberration.

JK1231

Actually, my glass lenses are about 5 times thicker than the plastic ones. (Think "coke bottle" lenses!)

I have rather substantial prisms in my glasses, and without them I have no depth perception during the day and double vision at night.

HipShot

I can't address the lens material question. But I wear contacts and, occasionally, glasses. My vision is pretty bad: 20/200+.

In my experience, the wider the lenses of my glasses are, the thicker they have to be. Have you considered or tried glasses that don't have as wide a field of view as your current ones? That would reduce the weight, I would think.

BabaGanoush

Heartyfisher said: i think the answer is its bec the plastic lenses need to be thicker to bend the light as much due to the lower index (density) hence the 2 surfaces are farther apart resulting in greater chromatic aberration.

Actually, it's the curvature of the optical surface(s) of the lens not its thickness that matters. What determines the angle of bending (refraction) relative to the normal to the optical surface is the ratio of the index of refraction of the optical material to the index of refraction of air. It's called Snell's Law. In the classical approach to optics, a light ray bends when it enters a lens of given index of refraction, travels on a straight line within the lens regardless of the physical thickness of the lens, then bends agains when it emerges from the lens. Perhaps the plastic lenses are not being molded into the proper shape for your extreme prescription.

Koru

Just ordered new glasses for myself. Yes, HipShot, you are correct. I cannot wear glasses that have a wide field of view as they quickly become very thick on the outside edge. My current ones are roughly 6.5 mm thick at the edge (15 year old plastic - top of the line in those days. 750 bucks) A further consequence is that the wider the lens gets, the thinner it becomes in the optical centre. My new lenses (newer plastic technology) will be about 10 percent thinner at the edge. I am pretty much limited to a "John Lennon" style and size.
JK1231, I feel for you. Glass gets so much thicker (and heavier!) than plastic. I have seen "coke bottle" lenses. I am very thankful for the plastic technology, but my optometrist agrees that Glass is "sharper" although it has its price (in weight/thickness).

PB_PM

Just make sure your glasses are made with Nikon glass! Mine are, even though I wear them infrequently.

BabaGanoush

Nikon glass? You cannot do better than Schott glass. Don't know whether their glass is used in eyeglass lenses, but B+W makes their filters from Schott glass.

PB_PM

Nikon glass?

Yes Nikon makes optical lenses for glasses, and has been doing so for many years.

Koru

Yes, but I believe their glass comes from Germany - at least it used to...
Anyway, there's no arguing that the world looks different through an empty Shott Glasss!
Sorry, couldn't resist!

sevencrossing

Just been searching the web

surprise surprise there are lots of different types of Plastic lens

Trivex is mentioned as the latest and greatest

Most UK opticians whist giving you the choice of Aurora,BOSS Orange, Cheap Monday,FCUK vision,French Connection, Gok Wan, John Rocha, Karen Millen, Osiris, Quiksilver, Red or Dead, Replay, Roxy, Timberland or Tommy Hilfiger, frames


The choice lenses is limed to, standard , or premium, no information what they are made of