Why do my lenses look like that?

Part of a comprehensive eye examination by one of our Optometrists involves vision testing and may result in a list of numbers, or prescription. The prescription may look something like this: -5.25/-0.75 x 85 or +3.00/-3.00 x 165. Just how is the prescription put into a lens?

1980’s Stage III Optics taught me lens material, lens thickness and lens surface curves could all be manipulated to make the prescription. It was logical and fun to design but didn’t account for the fact that all lenses have distortions and that eyeballs move. Stage IV Optics covered lens design and made my head hurt! Lens design considers the distortion (or lens aberrations) which are more obvious once the eye moves from the centre of the lens.

Modern lens design is used by our optical labs to produce lenses that are lighter in weight, thinner, less curved and with less distortion. You not only look good, you can “see good”.
Lenses were originally designed in glass which doesn’t scratch easily, (compared to other materials) but is heavy and not impact resistant. Nobody wants glass shafts in their eyes and you will find Optometrists and Dispensing Opticians reluctant to suggest them! Since the 1970s, plastic lenses have become more popular as they are lighter, safer and block more UV light, but they can be thicker and more easily scratched. A scratch resistant coating on plastic lenses reduces the risk of scratching but does not stop it. Fast forward another 10 years and polycarbonate was invented. Polycarbonate lenses are thinner and more impact resistant but have more aberrations and are softer. They must also have a scratch resistant coating. More modern Trivex lenses are even more impact resistant and, like polycarbonate, are perfect for children. Plastic (and glass) lenses can be made in denser or high index materials which are thinner but their weight, distortions and impact resistance can vary.

Lenses of all these types are sitting on laboratory shelves half made! Only one side of the lens, the base curve, is finished. Once the order for a lens is requested, the other side is “surfaced” to create the final prescription. The surface can be spherical, (think of a surface of a football), toroidal, (think rugby ball) or aspherical (the curve changes across the lens, think balloons, hearts and hydroslides ). Changing the surface of a lens can make lenses look slimmer and decrease the distortions. Again the aim is for you to look good and see “good”. Modern “digital design” lenses can now be made where both surfaces are enhanced for your particular measurements. Just like buying a new pair of shoes, new lenses, with different prescriptions or different curves can take some time to adjust to. If you feel a bit “off” with new lenses, it could be the lens aberrations that you are experiencing. Most of the time, you will adapt to it.

The frame that the lens fits into can also alter the appearance of your lenses. A frame that is big or small, flat or curved, thick rimmed or rimless may be just perfect or disastrous for your final look! When your Optometrist or Dispensing Optician is helping you choose your frame, they are considering your prescription as well as your taste and face shape so that you can look good and see “good”.