A commonly used contact lens design for people with presbyopia is the
concentric bifocal pattern. In this type of contact lens, the near
correction is in a small circle at the center of the lens, surrounded by a much
larger circle containing the distance correction. The distance correction could be
placed in the center instead, with the near prescription in the outer ring.
Beyond this familiar configuration, the two basic multifocal
contact lens designs are alternating and simultaneous image.
Alternating image designs (also called translating designs) have
distinct zones in the lens for distance vision and near vision. These designs are available
in gas permeable lens materials only.
Like bifocal eyeglasses, the top part of an alternating image multifocal GP lens is for
distance vision and the bottom part is for near. The two zones are separated by a nearly-invisible
line that helps your eyecare professional determine if the lens is fitting properly.
When you look straight ahead while wearing an alternating multifocal, you're looking
through
the distance portion of the GP lens. When you look down to read, the lens remains supported by
your lower lid, so your line of sight now passes through the lower (near vision)
portion of the lens.
The near segment can have a half-moon, crescent or annular shape.
(The annular segment circles around the entire periphery of the lens.) In alternating
multifocals
with half-moon or crescent-shaped near segments, the lens maintains its proper
rotational position by means of an area of unequal thickness in the lens called
a prism ballast. In some
cases,
the bottom edge of the lens is also truncated to help align it properly with the wearer's
lower lid.
Because alternating multifocal lenses typically have just two lens powers,
these lenses usually provide good vision for driving and for reading. But they may not perform
as well as simultaneous image designs for computer work and other intermediate-range
visual tasks.
Simultaneous image designs have both distance and near vision portions of the
lens in
front of the pupil at
the same time. These designs
are available in both soft and GP lens materials.
Your brain must
determine which area of the lens to emphasize and which area to ignore to provide
the best image resolution.
Blended designs, such as the aspheric example shown here, can be complicated because
they seek to correct the particular points of aberration in your eyes, to provide
the most natural
vision experience possible.