* Hard lenses are made of PMMA — also known as acrylic or Lucite. these lenses are virtually obsolete and rarely used.
* Soft lenses are made from plastic gel-like, containing water and are more common. They are slightly larger in size than your diaphragm.
* GP Lenses, also known as PSI or oxygen permeable "lenses", are made of rigid plastics, and are especially good for high astigmatism and presbyopia.These lenses are usually about eight millimetres in diameter, which is less than your diaphragm.
With the introduction of soft lenses, in 1971, until a few years ago, most brands of lens were made of plastic "hydrogel" then silicone hydrogel contact lenses were introduced. They made contact lenses of choice for many eye care professionals, because they allow more oxygen to pass through the lens of the eye, and they are less prone to dehydration.
