When the power of the lenses is small, normal index (i.e.
1.5) stock single vision lenses can be used as the thickness of
the lenses at the edges would be acceptable. The edge thickness
of the lenses is less when a smaller frame size is used. In the
higher powers of myopia, high index lenses are needed
to minimise the edge thickness of the lenses. Glass lenses have
been available up to 1.8 index for many years. However, it is only
in recent years that manufacturers have produced a variety of high
index lenses in plastic. Compared to glass, high index plastic
lenses are significantly lighter, are much more resistant to breakage
and unlike glass lenses, contain a UV filter. Plastic lenses now
come in a variety of indexes up to 1.74.