A cylindrical lens acts only in one axis; i.e. power is incorporated in one axis,the other axis having zero power. A cylindrical lens may be convex (plus) or concave (minus). A convex cylindrical lens is a segment of a cylinder of glass cut parallel to its axis, whereas a lens cast in a convex cylindrical mound is called concave cylindrical lens The axis of a cylindrical lens is parallel to that of the cylinder of which it is segment. the cylindrical lens has power only in the direction at right angle to the axis.
Therefore, the parallel rays of light after passing through a cylindrical lens do not come to a point but from a focal line
Identification of a cylindrical lens
1. When the cylindrical lens is rotated around its optical axis, the object seen through it becomes distorted.
2. The cylindrical lens acts in only one axis,so when it is moved up and down or sideways,the objects will move with lens (in concave cylinder) or opposite to the lens (in convex cylinder) only in one direction.
Uses of cylindrical lenses
1. Prescribed to correct astigmatism
2. As a cross cylinder used to check the refraction subjectively
3. Maddox rod consists of a series of powerful convex cylindrical lenses mounted together in a trial lens.
Images formed by cylindrical lenses
Cylindrical or astigmatic lens may be of three types:
1.Simple (curved in one meridian only, either convex or concave).
2.Compound ( curved unequally in both the meridia, either convex or concave).The compound cylindrical lens is also called spherocylinder.
3. Mixed cylinder,one meridian is convex and the other is concave.
The image formed by different types of cylindrical lenses are best understood by the study of Sturm`s conoid.
1. When the cylindrical lens is rotated around its optical axis, the object seen through it becomes distorted.
2. The cylindrical lens acts in only one axis,so when it is moved up and down or sideways,the objects will move with lens (in concave cylinder) or opposite to the lens (in convex cylinder) only in one direction.
Uses of cylindrical lenses
1. Prescribed to correct astigmatism
2. As a cross cylinder used to check the refraction subjectively
3. Maddox rod consists of a series of powerful convex cylindrical lenses mounted together in a trial lens.
Images formed by cylindrical lenses
Cylindrical or astigmatic lens may be of three types:
1.Simple (curved in one meridian only, either convex or concave).
2.Compound ( curved unequally in both the meridia, either convex or concave).The compound cylindrical lens is also called spherocylinder.
3. Mixed cylinder,one meridian is convex and the other is concave.
The image formed by different types of cylindrical lenses are best understood by the study of Sturm`s conoid.
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