As we get older, our vision fades. I’ve worn “no line” bifocals for many years. How do these things work? I know enough about optics to know that light refracts when it goes through a lens, but after closely examining my glasses, I cannot see any change in the curvature between the near and far sections.
You may not be able to see the change, but there is a change in curvature between the different parts of the lens.
Instead of cutting two separate pieces and fusing them together, which was how the old bifocals with the line were made, no-line bifocals are cut from a single lens using a computer-controlled 3-axis lathe. The change in curvature from one part of the lens to the other is gradual enough that it is not obvious when looking at the lens.
Actually, you can see a change in curvature because you can focus closer using the lower portion of your lens. If there were no change, your focus wouldn’t change either.
I’m being a little flippant; I think you mean that you can’t see the transition from one curvature to another. One reason the Transitions people chose their company name is because the big deal about lineless bifocals is the transition between curvatures.
In mathematical terms, both the near and far lenses exhibit continuous curvature. Obviously, those curves are different, so where they meet, there’s a discontinuity. That’s the line.
Lineless bifocals have a continuous intermediate curve that connects the two lens curves. It’s a compound curve involving multiple radii, so there’s some distortion at the transition, but no sharp line of discontinuity.
If you look at a mostly horizontal line and raise/lower your head, you should see the line get distorted when you’re in the transition zone between lens curvatures.
Transitions Optical makes photochromic lenses, which darken automatically in bright light, not no-line bifocals.
“No Line” bifocals are properly called Progressive Lenses. There is a significant departure from a spherical shape, but it’s not discontinuous in the way traditional bificals are:
It’s hard for me to notice the change from one part of the lens to the other now, but it wasn’t when I first got them. During that time, I once walked up a driveway with a truck parked in it. At one point it looked like the truck moved a foot or so towards me with some distortion of the truck’s image in the process. I later figured out that that was an effect of the image moving from the distance part to the up close part.
BTW, the reason one needs bifocals or reading glasses when one gets older is that the lens of the eye stiffens with age, so the eye muscles can’t focus them correctly. This is probably caused by proteins cross-linking with each other, one of the reasons arteries and some other body parts get stiff with age.
Well at least SOME body parts get stiff with age. (Sigh)