The lenses you choose for your glasses (even more than the frames) will often determine how satisfied you are with them.And buying eyewear lenses is no easy task. In fact, Consumer Reports magazine says in a recent issue, "With so many choices of lenses and coatings, it's easy to get confused about what's worth buying."
This buying guide will help you take the hype out of the different types of eyeglass lenses and help you choose eyewear and coatings that offer the best function and value.
When buying glasses, the frame you choose is crucial to the way you look and the comfort you feel when wearing them. But your choice of spectacle lenses affects four factors: appearance, comfort, vision and safety.
A common mistake people often make when buying glasses is not taking enough time to consider their choice of eyewear lens materials, design and coatings.
The Abbe value (or Abbe number) of a lens material is an objective measure of how well a lens disperses light of different wavelengths as it passes through it. A lens material with a low Abbe value has a high dispersion, resulting in significant chromatic aberration - an optical error that is visible as a coloured halo around objects (especially lights).
When present, chromatic aberrations are most pronounced when viewed from the periphery of the spectacle lens. It is least noticeable when viewed directly through the central optical area of the lens.
The Abbe values of spectacle lens materials range from a high of 59 (crown glass) to a low of 30 (polycarbonate). The lower the Abbe number, the greater the likelihood that the lens material will cause chromatic aberrations.
The Abbe number is named after the German physicist Ernst Abbe (1840-1905), who defined this useful measure of optical quality.
In addition to choosing a lens material with a high refractive index, another way to make lenses slimmer and more attractive in profile is to opt for an aspheric design.
The aspherical lens design - where the curvature of the lens changes gradually from the centre to the edge of the lens - allows lens manufacturers to use flatter curves in the manufacture of spectacle lenses without reducing the optical performance of the lenses.
Because aspherical lenses are flatter than conventional (spherical) lens designs, they reduce unnecessary magnification of the wearer's eyes, resulting in a better appearance. In some cases, aspheric designs can also improve the clarity of the wearer's peripheral vision.
Most high index plastic lenses are aspheric in design to optimise the appearance and optical performance of the lens. For polycarbonate and CR-39 lenses, aspheric designs are usually chosen, which can increase the cost of the lenses.
The FDA has guidelines for impact resistance, so there is a limit to the thickness of lenses that can be ground by optical laboratories.
In (concave) lenses used to correct nearsightedness, the thinnest part of the lens is the optical centre, located in the middle or middle. In (convex) lenses for the correction of distance vision, the thinnest part of the lens is at its edge.
Thanks to the excellent impact resistance of polycarbonate and Trivex lenses, it is possible to manufacture lenses for the correction of myopia to a central thickness of only 1.0 mm and still pass the FDA impact resistance criteria. Myopia-correcting lenses made from other materials usually have to be thicker at the centre to pass the standard.
The size and shape of the frame will also affect the thickness of the lenses, especially if you have a strong prescription. Whichever lens material you choose, choosing a smaller, centred frame can significantly reduce the thickness and weight of your lenses.
Typically, the thinnest lenses for prescriptions will be aspheric lenses made from high refractive index materials and worn in small frames.