W5 Astigmatic Lens Forms

Why are meniscus lenses used?

Better image quality, especially in the lens periphery

What is a toric lens?

A lens that has different powers in different directions to correct an astigmatic prescription

What is the base and cross curve on a toroidal surface?

The base curve is the FLATTEST curve with the LOWEST power

The cross curve is the STEEPEST curve with the highest power

What are the 3 toroidal surfaces used in ophthalmic lenses?

Capstan formation, barrel formation and tyre formation

What is the structure of a tyre formation?

The lens has different curvatures in two perpendicular directions SO the light rays in each direction focus at different distances rather than a single point. As a result, the image formed is tyre/ring shaped

What is the structure of barrel formation?

Light rays in one direction focus closer to the lens while rays in the other direction focus further away due to the difference in curvature in toroidal lenses in 2 perpendicular directions. This forms a barrel shaped image rather than a single point

What is the structure of capstan formation?

Light focuses more on the outer parts and less in the centre, this makes the image look narrow in the middle and wider at the ends. This forms a capstan image rather than a single point due to unequal curvature and focusing

What are features of the tyre formation?

Axis of rotation and the vertex of the generating arc lie on opposite sides of the centre of curvature of the generating arc

What are features of the barrel formation?

Axis of rotation lies between the vertex of the generating arc and the centre of curvature

What are features of the capstan formation?

Concave in one meridian, convex in the second principal meridian

Axis of rotation and centre of curvature of the generating arc lie on opposite sides of the vertex of the generating arc

What are uses of the tyre formation?

Mass produced, most common surface

What are uses of the barrel formation?

Good axis performance- not suited for mass production

What are uses of capstan formation?

Rarely used- sometimes used for solid bifocals

How do you transpose from cross cylinder form to -ve sphere cylinder form?

Sphere will be most positive cylinder

Cylinder is most negative minus the positive

Use axis of the least positive cyl