Seeing double.Since contact lenses move with your eyes as they move, how are bifocal contact lenses possible?

One bifocal contact lens design—called alternating, or translating,
vision—is similar to bifocal glasses. Each lens has two segments. The distance
correction is on top, and the near correction is below. The eye
moves between the two lens powers as the gaze shifts up and down.
Conversely, simultaneous-vision lenses are designed so that the eyes
look through both near and distance powers at once, and the visual system
determines which power to use.
Alternating-bifocal contact lenses can be weighted, or slightly flattened
at the base, so that the lens is supported by the lower lid and is
shifted upward relative to the pupil when the gaze is directed downward.
The simplest form of simultaneous vision is monovision. One eye,
usually the dominant one, is fitted with the distance correction, and the
other eye is fitted with the near correction. More complicated designs
are concentric ring lenses and aspheric lenses. Concentric ring designs
feature a bull’s-eye pattern of the near and far prescriptions. Aspheric
designs have the two powers blended across the lens.
Because simultaneous-vision lenses maintain both the near and far
prescription powers in front of the pupil at all times, both powers focus
light onto the retina. Therefore, the retina receives two images—one
that is in focus and one that is out of focus. Over time, the brain learns to
make sense of this strange state of affairs by paying attention to the clear
image and ignoring the superimposed out-of-focus image.
The adaptation is not perfect. Monovision reduces depth perception
because only one eye receives a clear image of any scene. Simultaneous
lenses with more than one power per lens reduce visual acuity—the
sharpness of an image—because the out-of-focus image creates a veiling
effect on the retina.
Another problem with bifocal contact lenses is that the way lenses fit
over an individual’s cornea is unique. As a result, it is not easy to predict
where the optical center of the lens will be and whether the power zones
will line up correctly with the pupil.

Because of these challenges, bifocal contact lenses are not as popular
as single-prescription lenses. However, the technology has improved,
and there are more designs to choose from. Which design is best for an
individual depends on the shape of the eye as well as a person’s lifestyle
and activities.

published 2 years ago

Gabriel

213 points