“These different designs for bifocal contact lenses are called simultaneous, concentric and alternating vision styles. Simultaneous vision designs fit centered on your cornea with both the distance and near prescription within your pupil area. Your own visual system learns to interpret the correct power choice depending on how close or far you’re looking. In a concentric design, the center portion of the lens has the distance power and the outside or periphery portion has the near power, or vice versa. Alternating vision designs work much like a bifocal eyeglass with a line in that the top of the lens has one power and the bottom has the other.”
Go to the above site for further explanation, diagrams and for more links regarding this.
Thanks for the link, I have often wondered this myself. But I’m not sure if I’m understanding it correctly. It sounds to me like whatever you are looking at directly will be in focus if it is in one distance range, and peripheral vision is focused for the other distance range. This sounds dumb to me, like it is not much different than non-bifocals. Who cares if the periphery is focused – you’re never looking at the periphery!
In case the above was not clear, I’ll rephrase it into a direct question:
With such a design, how does one read a book? It sounds to me like you would have to point your eyes at a line other than the one you’re trying to read, because whatever your eyes are pointed at will be in focus only if it is far away!
Your own visual system learns to interpret the correct power choice depending on how close or far you’re looking.
I.e., your brain makes sense of a mess of two different prescriptions, alternating on the same lens. If you’re asking HOW the brain does it, I’d guess that nobody knows! I wonder how long it takes for the brain to learn to do that, when you first get the contacts.
Maybe the question now is- if the brain can merge bifocal contacts so that it handles two prescriptions at once, why can’t it do that with bifocal glasses? Is it because the glasses are too far from the eye?
I think I get it now- It’s like looking at a MagicEye® poster.
You see what you concentrate on and the rest blurs into the background.
Your own human lens can change focus as you concentrate, bringing one of the two images into focus and blurring the other so it’s just background noise.
Mushrooms always grow in damp places … and so that is why they look like bumbershoots.
T-shirts, I recently tried a pair of Acuvue bifocals (in fact, I tried twice) but gave up. I found I could see clearly neither near nor far, and frankly, I too was mystified as to how they were supposed to work. No one explained the “brain training” period required – even so, my eyes ached so badly at the end of the day, I gave up on them. Now I wear “distance” contacts and use reading glasses as necessary - and my eyes feel a lot better for it. Besides, I can simulate the appearance of wisdom by peering over the top of my reading glasses. I even impress myself.
Thanks for the warning- nothing substitutes for experience.
Oh, well. I’m on “monovision” Acuvue extended wear contacts now (which I love the feel of). One eye corrected for near, and one for far. I can read and drive and the brain sorts it out, but oddly I need a monocle (cheap +1.0 reading glasses with one lens popped out) for reading the computer screen.
People keep trying to talk me into radial or non-radial corneal surgery, but that only corrects for near or far, not both.
Dangit all this time I thought the contacts had some sort of weighting that ensured that the bi-focal part always ended up at the bottom of the eye and then you just looked down to see out of the different perscription area.
Oh well. I don’t wear contacts and cannot imagine putting them in my eyes.