The different types of eye defects are
1) Myopia : The defect of the people who cannot see the objects at far distances but can see near objects clearly is called myopia (or) Near sightedness.
It can be corrected by using a concare lens of suitable focal length.
2) Hypermetropia : The deject of the people who cannot see the objects at near distances. This is also known as 'far sigtednes'. This can be corrected by using a convex lens of suitable focal length.
3) Presbyopia : The decreasing nature of ability of accomodation of the eye with ageing is known as presbyopia. To correct this type of deject of vision we need bi-focal lenses which are formed using both concave and convex lenses. Its upper portion consists of the concave lens and lower portion consists of the convex lens.
4) Astigmatism : If a distorted or fuzzy vision on any distance, it can be a symptom of astigmatism. Even in case, that your vision is sharp, but you have head-ache, tired, squinting or irritated eyes, it can be a symptom of weak astigmatism. It can be found by eye examination.
It can be corrected by lenses called foric and instead of having one steric area they have one cylindrical area. The bend of these lenses compensate cornea bend. Usually every cornea has astigmatism. If it is weak or fryziologic it is not correct-ed. Minimum value of dioptres to correct astigmation is 0.25.
5) Colour blindness : It is because of colour responding rod cells in the retina and also due to genetic dissorder. If is not actual blindness but deficiency of vision.
6) Night blindness : Night blindness is also called as 'nyctalopia' is the inability to see well at night or in poor light. If is not a disease but rather a symptom of an underlying disorder or problem especially untreated near sightedness.
7) Hemeralopia : It is the opposite problem of night blindness the inability to see in the brightness and is much rarer.