Contact lenses are available as bifocal, trifocal
Basically, bifocal contact lenses are prescribed for those with Presbyopia, a natural condition that develops due to age. The eye's lens loses its flexibility, making it difficult to focus on nearby objects. Some people use multi-focal, which have a range of powers just like the progressive design. With bifocal, two prescriptions are on the same lens.
So how do they work?
Based on the lens' design, bifocals can work in a number of ways. The design can be alternating vision where the pupil has to handle two powers and simultaneous vision where the eye must view distance and near powers at once. The eye manages to choose the right power depending on what it needs to see.
Simultaneous lenses can be concentric or aspheric design. In concentric design, distance power is at the center with near and distance power in the outer rings. These are also available with near power in the center. In the aspheric design, both the near and distance power are in front of the pupil.
What about alternating bifocal contact lenses?
These are quite similar to the bifocal eyeglasses and come with two power sections. You can see the demarcation between the distance power on top and near power under it. Your eye chooses the area of lens to see through, based on what you want to see. Bifocal lenses, like eyeglasses, also stay in place when you look downward or upward correcting your vision.
The concentric ring design has a prescription at the center with a couple of rings with power around it. There will be at least two rings in the pupil area, changing as the pupil expands or contracts with changing light conditions. These can be soft or rigid with varying locations for the power.
Although these contact lenses have been around for a while, advances in the technology have made it more effective than before, offering the patient more choice.
Finding the right choice for you
Even if you only need reading glasses, you can get bifocal contact lenses with only the reading prescription, making it convenient. Even people with astigmatism can obtain GP lenses, which stay in place and help correct the vision.
To identify the right type, your optician will take into account the size of your pupil and near vision power. Different designs may be suggested before the right one is found and many opticians offer free trials. It is best to consult with a qualified and experienced eye doctor to diagnose your needs and find the best fit for you.