Corrective Lenses
Corrective contact lenses are designed to improve vision by correcting the refractive error, which is an error in the focusing of light by the eye. This is accomplished by focusing light so that it enters the eye directly with the proper power for clear vision.
A spherical contact lens bends light evenly in every direction (horizontally, vertically, etc.). These type of lenses are usually used to correct nearsightedness and farsightedness. Nearsightedness occurs when light that comes in does not directly focus on the retina but in front of it. This causes the image when looking at a distant object to be out of focus, but in focus when looking at a close object. Farsightedness occurs when there is an imperfection of the eye. This results in not being able to clearly see objects that are close by.
A spherical contact lens bends light evenly in every direction (horizontally, vertically, etc.). These type of lenses are usually used to correct nearsightedness and farsightedness. Nearsightedness occurs when light that comes in does not directly focus on the retina but in front of it. This causes the image when looking at a distant object to be out of focus, but in focus when looking at a close object. Farsightedness occurs when there is an imperfection of the eye. This results in not being able to clearly see objects that are close by.
Farsightedness (top) and restoring vision with a convex lens (bottom). Nearsightedness
The correction of presbyopia can also be dealt with contact lenses. Presbyopia is a condition where the need for a reading prescription is different from the prescription needed for distance. Currently two main strategies exist. Multifocal contact lenses and monovision. Multifocal contact lenses are like bifocal lenses as they have multiple focal points. Multifocal contact lenses are designed for constant viewing through the center of the lens. Monovision is the use of single vision lenses (one focal point per lens). This focuses one eye for distance vision and the other eye for near related work. The brain eventually learns to use this setup to see clearly at all distances.