Victor, thank you. You're giving me the most serious debate I've had here in days.
This question comes up often enough to be a FAQ. Plastics are cheap, and they're so much easier to grind to lens shapes than goass, so you'd think they'd be in common use.
Or mold. I think a fair number of lenses have plastic components in them.
Only in the form of the "hybrid aspherics" I mentioned earlier. I haven't seen a serious, major maker lens with a full plastic element in anything other than a point and shoot in decades.
I have a cheap preset 400mm lens (don't use it anymore) that has a plastic lens. After a few years, it started having some issues. I disassembled the lens and buffed that piece with a soft cloth, improving it substantially.
Ick. Yeah, that's a big difference. Plastics are hydrocarbons, with relatively low bonding energies, and molds can take them apart to make more molds. There are some molds that can "feed" off glass, after a fashion, but the bonds are higher energy, and all they can really do is go after the oxygen, leaving the silicon behind.
Eyeglasses are slow, 2 diopter (average strength) glasses 60mm across are only f9.
So what? They're supplemental lenses. A Canon 500D closeup lens with a 58mm thread would also be an f8 lens. That doesn't really mean much for a supplementary lens, though.
Good point. That comes down to two things. The Canon 500D is often used with huge pupils, like 70-200mm f2.8 lenses (71mm), so you actually see the supplemental lens's speed. The small pupil of an eye "stops" the glasses down to about f50.
They sit in front of the human eye, which is not color corrected and has terrible CA, not to mention very high distortion, and requires a huge computer to correct. Photographic lenses are shorter focal lengths, and much faster, and they have to have their CA and distortion much better corrected than human vision. And eyeglasses are single elements, while photographic lenses have up to 28.
You really should be comparing the eye (rather than eyeglasses) to a photographic lens, it seems to me.
That is true, since the eye is a lightweight organic, relatively slow, uncorrected system that relies on the brain to "fix" the horrid images. But Ho72 brought up eyeglasses as the example of why this should be possible.
So, while plastics are suitable for eyeglasses, they're not suitable for most photographic lenses for several reasons...
- Glass, amorphous SiO2, has a molecular weight of 60.1, while plastics stable enough to hold shape have molecular weights in the hundreds of thousands or millions. So, when photons go through plastics, their paths get altered passing through forests of those huge polymer molecules. This limits resolution and causes a certain "veiling" effect.
Isn't a lens' job to alter the light's path?
Har har....
OK, maybe I should have been clearer. The optical polymers alter the path of photons in a random fashion, causing the light to lose focus even if the optical design is good. They "scatter" light.
- Different glass formulas give you a range of dispersion (change in focal length of a lens with different colors of light, the cause of "chromatic aberration) from Abbe number around 20 (very high dispersion) to 100 (extra low dispersion). By combining positive and negative elements of different dispersions, you can cancel chromatic aberrations. There's no such thing as ED plastic.
... which doesn't mean you can't use them for
some of the elements.
True. Once you get past the scattering effects, the difficulty coating, and the mold.
- Plastics, even cross-linked polymers, "creep" after they're cast, ground, or mounted. After it ages a few months or years, a plastic lens no longer has a shape whose precision is measured in fractions of a wavelenght of light, and it loses resolution.
... unless it is cemented to a glass element where it has no place to creep.
That would stabilize one surface, but unless the plastic was very thin, it's not stabilizing the other surface. And if the plastic is very thin, you can't do CA compensation, and it's not saving much weight.
There's another thing. Look at modern lens designs, the number of cemented elements goes down every year. The main reason for cementing elements was reflection. Modern optical coatings reduce that. Two cemented elements give a lens designer 6 degrees of freedom, the position and radius of curvature of the front, rear, and shared surfaces. Not cementing them gives the lens designer 8 degrees of freedom, the position and location of 4 independent surfaces.
This, in and of itself, can be used to reduce the weight of lenses, as 3 independent elements (6 surfaces) can now do the work of 4 elements cemented in pairs (also 6 surfaces).
(stupid 6000 character limit!)
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Rahon Klavanian 1912-2008.
Armenian genocide survivor, amazing cook, scrabble master, and loving grandmother. You will be missed.
Ciao! Joseph
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