Multifocal eyeglass lenses contain two or more lens powers to help you see objects at all distances after you lose the ability to naturally change the focus of your eyes due to age (presbyopia).
RT-28 lenses has two different lens powers that aid in seeing objects at various distances once the aging process reduces your ability to do so.
Round top bifocals are further described by the width of the near segment, measured in millimeters (mm). The most popular RT bifocal is the RT-28, a round top bifocal with a near seg that is 28 mm wide.
Whatever the reason one might need to obtain prescription for correction of near-vision, bifocals work similarly. That is, the small lower lens segment (also called "seg") has the refractive power you require to correct near vision while the other part of the lens works for distance vision.
| || |
In using bifocals, you would look through the upper part of the bifocals to focus on farther objects and look down through the lower segment to read or focus on objects at a distance of 18 inches or less.
An advantage of round seg bifocals is that the line separating the distance and near zones of the lens tends to be less noticeable than the upper line of a D-seg bifocal
Bifocals entail the sacrifice of part of the distance field of vision. Problems encountered in the beginning, range from acute nausea and actual vomiting to virtually no discomfort.
Part of the problem for a new bifocal wearer is to learn to walk with the road slightly out of focus and to get down staircases bending the head down a little more than usual. Reading on the other hand requires an opposite action. For the first time in his visual experience, he will read without tilting his head down but only turning his eyes down.
Again, no longer having a complete segment (unless he uses an executive bifocal) to read from he now has to turn his head to follow the lines. If he scans a ledger, the entire head has to be tilted up or the ledger brought down to start reading.
To be fair, a good proportion of wearers can adapt to a bifocal in about 3 weeks' time.