Glossary of Lens Terms
We hope that this will aid you in making an informed decision and understanding the terminology associated with lenses.
Single Vision Lenses
These lenses have one focal point. They can be used for driving, computer use, or reading. After age 45 you may require more than one set of lenses for different visual tasks.
These lenses have more than one focal point. This can be found in lined bifocals, lined trifocals, or progressive lenses.
These have 2 (bifocals) or 3 (trifocals) focal points. Typically, they are designed for your distance and reading vision (bifocals) or for distance, intermediate, and reading vision (trifocals). They can also be designed for intermediate and reading (eg. computer distance
in the top half and desktop distance in the bottom half). They have a wider field of view; however, because of the line; they have a limited range in depth of focus.
This will reduce the amount of glare or reflections that come off of oncoming traffic, overhead lighting, computer screens, etc. It will allow you to see more comfortably. We recommend an anti-reflective coating if you use your glasses for driving. There are several different types of anti-reflective coatings. The newer coatings have anti-static characteristics that will make them much easier to clean with fewer smudges. However, you will still need to clean your glasses.
Also known as no-line bifocals. These allow for distance, intermediate, and near vision. They are ground in such a way that there is no line in the lenses, but in order to do this, there are peripheral distortions in the lenses. This cannot be avoided. As a result, they offer a greater range in vision but a narrower field of view. Some types of newer digitally surfaced progressive lenses offer a wider field of view.
Plastic vs. Glass vs. Polycarbonate
Most lenses used today are plastic lenses. Plastic offers the best combination of strength, scratch resistance, and cost. Glass lenses are the most scratch-resistant but the least impact resistant. Glass is also the heaviest material. Polycarbonate is used in certified safety glasses. It is the softest material which makes it the most impact-resistant, but this same characteristic allows it to scratch very easily.
This refers to how much a material will bend light. Higher index lenses will bend light more, allowing for thinner and lighter lenses. Typically any prescription over +/- 3.00 we will recommend a higher index material. Any prescription over +/- 4.00 we strongly encourage you to consider a higher index material in order to avoid thick and heavy lenses.
Photochromatic Lenses (aka Transition Lenses)
These will change color in response to UV exposure – these lenses will darken (e.g. outdoors). With indirect UV, they will darken, but not as much (e.g. in a vehicle). They return to clear when indoors. These lenses typically darken very quickly and take longer to lighten up. As they age (after 2-3 years) they become less chemically active and they do not turn as light or as dark. This is normal. Also, cold weather can affect how chemically active they are. If you are looking for dark glasses for driving purposes, be aware that these lenses will not darken as much in a vehicle. These lenses are often confused with progressive lenses.