Bifocal contacts are very much like bifocal glasses, as they serve two interests simultaneously; the first correcting near vision and the other distant vision. Since the lens has two separate functions, it can be designed in various ways. These different designs for bifocal contact lenses are called simultaneous, concentric and alternating vision styles. While some prescription bifocal contacts place the near and far corrective parts on the same part of the lens, other designs place them away from each other, so that you must flick your eyes in the right direction of the lens to utilize the prescription. Bifocal glasses are usually near prescription on the bottom with the distal prescription on the top. Contact lenses aren't as restrictive in this and you can work with your eye practitioner to find a design that suits you perfectly. Your eye learns to differentiate the proper power for the correct distance.
In a translating lens , the distal prescription is on the top half of the lens so that you can look up and away, while the near prescription is on the bottom so that so can look down to read a book, for example. The bottom of the lens is also flattened instead of ballasted so that it doesn't move while the eye rotates. This is very similar to bifocal eye glasses and though it takes a little time to get used to, it is very effective and eventually comfortable.
In the aspheric bifocal lenses , the corrective part of the lens in very near the center, almost at the pupil. Both near and distant vision are corrected near the pupil. This is a simultaneous vision devise fit centered on your cornea with both the distance and near prescription within your pupil area, allowing your own visual system to interpret the correct power choice depending on how close or far you're looking.
In a concentric lens , the near and far prescription parts of the lens aren't that close together, as the distant visual correction is closer to the perimeter of the lens while the near corrective prescription is near the center. This is an alternating vision design that works much like a bifocal eyeglass with a line in that the top of the lens has one power and the bottom has the other.