It’s a common, unavoidable side effect of normal aging. The lenses of your eye stiffen and lose flexibility, making it harder to focus on objects at a close range. This condition is known as presbyopia.
Condition in which the aging eye, beginning at around age 40, is unable to focus at all distances, often noticed when print begins to blur. Additional symptoms include eye strain, headaches, and squinting.
Presbyopia affects more than a billion people around the globe. Luckily, for those with access to basic eye care, presbyopia is easily remedied by use of eye aids such as glasses with bifocal lenses. Bifocals offer a “split screen” combination of lenses, allowing users to use to the top portion of the lens for distance and a segment of the lower part for up-close objects. Any discussion of bifocals leads to another definition:
The shift of image that an individual experiences when passing from the distance portion of a lens into the magnifying segment area in a lined bifocal.
Image jump occurs because of the hard border between lenses, and the rapid change in image the eye experiences as it crosses back and forth over this border. To reduce or eliminate the jump, one must reduce or eliminate the border.
Some bifocals are better than others at controlling image jump. A bifocal with a softer line between lenses will typically cause less stress for the eye when it crosses back and forth over that line. A hard line requires a sharper, more abrupt transition, and can be jarring for those who are not accustomed to the effect.
One way to eliminate image jump completely is to opt for a progressive lens. Progressive lenses do not utilize a line at all; they transition smoothly between various lens powers, allowing your eye to adjust as it moves. They allow for a gradual change in prescription strength as opposed to the “all or nothing” approach of a bifocal.
Image jump is not always a problem for bifocal users. Some people don’t really mind the effect while others simply learn to live with it. If you’re thinking about a bifocal purchase to compensate for your onset of presbyopia, we suggest you try out a few generic pairs before “taking the plunge.” If the image jump effect causes you discomfort, undue annoyance, or nausea, you may want to consider progressive lenses instead.