Question:
How do bifocal/multifocal contact lenses work? Neither my optometrist's
explanation nor the explanation that was published recently in Scientific
American makes optical sense unless the lens also slides appropriately over
the eyeball ... and even then it is hard to imagine how it could be
engineered to work very well or very reliably.
Please, I can speculate as well as you can; I'd like to hear from people who
know how it is actually done.
Answer:
Multifocal/Bifocal contact lens designs.
There are a number of designs and many variations on a theme. Rigid lenses
can be "translating types": the optical axis passing through the middle
of the lens for distance optics and a place closer to the edge for near
optics. The lens moves up as you look down, causing the line of sight to
pass through or "translate to" the near segment. This is very similar to
using spectacle bifocals. Some of these lenses have a flat-top near
segment, some are crescent shaped, some divide the lens into two halves.
The near segment power is achieved by adjusting the front surface
curvature and/or using an imbedded segment of a different refractive
index. The lens is stablized to "ride" to the correct positions by
varying the edge thickness to create a ballast, or weight, and/or by
truncating (slicing off) the bottom edge of the lens, and/or using the
lower eyelid to support the lens.
Another design, available in soft and rigid material, is called "back or
multi aspheric". The lens produces a constantly varying power from the
center radially toward the edge. Some designs position the distance optics
in the center of the lens and others place the near power zone in the
center. Optically, the varying power creates a multifocal system, unlike
the translating bifocal lenses. This is similar to PAL spectacle lenses,
also known as "progressive" or "no line" bi focals.
