I’ve always wondered why it is that if I take my glasses off and look though the lenses backwards (with the ear pieces sticking away from my head) I can still see almost compleatly clearly. I always thought that they correct your eyesight with the curvature of the lens. Does it have more to do with the thickness or am I missing something all together?
I assume you’re either near-sighted or far-sighted. Astigmatic lenses can be more complicated.
The feature that really matters is whether the lens is convex (thicker in the middle than at the edges, used for far-sighted people) or concave (thinner in the middle than at the edges, used for near-sighted people). This essential feature is the same regardless of which direction you look from. True, the lenses are asymmetrical: they bulge out, away from your eyes, unlike the “straight” up and down profile of a magnifying glass lens. But that bulging curvature is to allow room for your eyelids and eyelashes to move without rubbing the lenses. It isn’t part of the optical correction.
>that bulging curvature is to allow room for your eyelids and eyelashes to move without rubbing the lenses. It isn’t part of the optical correction.
Actually, it is part of the correction. Lenses generally add less abberation when their middles (ie the imaginary surface inside that is equidistant from the two real surfaces) are curved so that the light rays they are to bend intersect the middles at similar angles. Said another way, you want each light ray the lens handles to enter and leave the glass with about the same angles relative to the entering and leaving surface. So, for example, a plano-convex single element lens held with its flat side towards a screen and its curved side towards some scenery will make a better image than will a convex-convex lens of the same focal length and same glass. The degree to which a lens bulges away from its finite conjugate and toward its infinite conjugate would be pretty much the first thing you would tune to try to improve the image sharpness, once you had decided on a focal length and diameter for the lens. Take apart a camera lens - you’ll find many of the telements are curved, and there’s usually a pattern to the curving (such as all the elements being concave toward the iris or aperture stop, wherever in the lens that happens to be).
Thanks for the education. I wasn’t aware of that.
Significantly nearsighted (as in, pretty much can’t read the big E on the eye chart), astigmatic Doper here. When I look through my glasses backwards, I can still sort of see, but everything’s a bit smaller and I get very nauseus due to the unfamiliar magnification.
On a slight hijack, I’ve noticed that my glasses give a colored tint to the edges of objects, blue on one side, and red on another…
Empirical evidence coming up:
I can read this thread with my glasses on.
I can turn my specs 180° on the vertical (looking through them backwards through the proper lenses) and read it.
I can turn them 180° on the horizontal (just turning them around like I was cleaning them, but peeked through them). Wrong eye and backward on each lens, unlike the above. I can still read the thread. Even better than the example above.
If I take them off, the thread looks like stewed tomatoes. I don’t know how they do it, but it works. FWIW, it’s very clear but microscopically reduced in example #1, like looking through the back of some binoculars or something. Example #2 yields perfectly fine vision.
Going back to Napier’s bit on abberations, there is a visually significant difference in the curvature of your visual field when you turn your glasses around.
To see it, turn your head with your glasses on normally, and notice how nice and flat everything looks. Now reverse your glasses and turn your head, things that look straight at the center of the field will look curved as they move toward the edges. The effect actually happens regardless of which way you orient the lenses, but your visual system has learned to compensate for the curvature when the lenses are worn right way out.
That’s known as chromatic aberration. Essentially, the index of refraction of the lens material varies slightly over the visible spectrum, so blue light is focused differently from red light.