from The American Heritage® Dictionary of the English Language, 4th Edition
- adj. Having three focal lengths.
- adj. Having one section that corrects for distant vision, a second section that corrects for medium vision, and a third that corrects for near vision, as an eyeglass lens.
- n. A lens having three focal lengths.
- n. Eyeglasses having trifocal lenses.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- adj. Of or pertaining to corrective lenses that have three different powers per eye.
Sorry, no etymologies found.
The cost, complete with two pairs of progressive (trifocal blended) lenses (one sunglass type) and the frames was around $2500 pesos.
Since I also tend to shoot a rifle with both eyes open, I'm thinking about having a set of lenses made with the right lens the same prescription as the center lens of my trifocal to see the sights clearly and the left lens the same prescription as my top lens for distance.
I just tilt my head up a little to look through the middle lens of the trifocal.
I also have a problem with my vision: I had to switch over to wearing trifocal glasses 2 years ago.
People who wear trifocal or progressive lenses often find that the middle-distance portion is too small to use the computer comfortably; they end up constantly moving their heads, and the screen can seem wavy or out of focus.
His once keen vision had fallen victim to endless hours of peering at word processor screens, so he now sported heavy trifocal contact lenses that made his eyes seem slightly bugged out, like a frog's.
Only bifocal and trifocal lenses were available in the past.
The downside of trifocal lenses is limited continuity of vision and peripheral distortion greater than with bifocals or single vision lenses.
A trifocal variant is occupational "readables" with a relatively larger center zone for mid-range computer distance and proportionately smaller lower and upper zones for reading/close work and focusing at about 10 feet for "room-type vision" respectively.
A "do-all" solution is trifocal glasses with three lens planes combining an upper segment for distance vision, a lower/bottom one for close work, and a third one for mid-range or screen distance in between.