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- (minus) lenses / myopia
To correct near-sightedness or myopia. .
The eye is too long / the eye lens is too curved (correcting too much).
+ (plus)glazen / hypermetropie
To correct far-sightedness or hyperopia
The eye is too short / the eye lens is not curved enough (correcting too little).
The eye is spherical, like a round ball.
The correction that is needed for myopia or hyperopia is the same for all angles of inclination. The corrective lenses used are spherical. When spherical lenses are rotated in front of the eye the image stays the same. Lenses in binoculars, camera's, magnifying lenses etc. are in principle all spherical.
The eye is thoric shaped, like a rugby ball.
Astigmatism is a difference in corrective capability of the eye in a certain direction (axis). With a rugby ball shape the curves of the 'long' side and 'short' side are different. When not corrected the eye will see better in one direction than the other and deform shapes, making them small and long for instance. It has to be corrected with thorical lensesby adding a cylindrical shape to a spherical lens. When a thoric lens is rotated in front of the eye the image will change. The right degree of the axial rotation of the cylidrical shape is of major importance to have optimal vision. Astigmatism can be present in combination with both myopia and hyperopia and is almost always the same for far or nearsightedness.
Minus(-) cylinders can be converted to plus(+) cylinders and visa versa.