Buying glasses? What to look for in the way of lens options and coatings.
When buying glasses, you're likely to notice various coatings offered. Some retailers include them for free. Others charge. They're pretty self-expanatory, but here's a quick rundown of what they are.
Take a piece of clear glass and hold it between you and the computer screen. Now look at the screen through that glass. If you have lights on in the house, or sun coming through the window, you'll probably see any light sources being reflected by that glass, impairing your vision.
Glasses reflect too. Fortunately, they've created an anti-reflective coating for glasses that in most cases eliminates any reflections. If you have glasses that already have an AR coating, taking them off and holding them at an angle to the light will probably show a light green transparant color - this is the AR coating.
AR coatings generally result in much clearer vision. There is a side effect to AR coatings however... When glasses get dirty or smudged, any dirt/smudge is very obvious through an AR-coated lens. If you keep your glasses clean however, you won't notice this.
When you buy glasses, some retailers include the AR coating for free, or for a small fee (around $5). Others add up to $25 for an AR coating.
An anti-scratch coating is typically used on CR-39 and Polycarbonate lenses. Since both of these lens types can be scratched (polycarbonate to a lesser degree), it simply adds a hard coating that helps to reduce the chance of this happening. Usually, it will protect from simple things that tend to leave minor scratches/scuffs (such as laying your glasses down on the table lens-first). It certainly won't protect from anything major, but for the common things that tend to leave scuffs and scratches, it helps quite nicely.
Most retailers include anti-scratch coatings for free.
UV coatings help to protect your eyes from the sun. It's beneficial to your eye's long-term health to have a UV coating applied to your glasses. This doesn't save your eyes from damage if you decide to stare directly at the sun (so don't be silly), but it should reduce the amount of UV rays that get to your eyes over the long-term. Polycarbonate has inherent UV-protection built-in, but wearers of other lenses would certainly be wise to get this coating.
Many retailers include a UV coating for free when you buy glasses. Note that some retailers do *not* include a separate UV coating with tinted lenses, so you may want to inquire individually as to whether their tint includes UV protection.
Tints are offered by some retailers in varying colors and at various intensities. They're often used for fashion purposes, but some wearers choose them to reduce the brightness when out in the sun. Note that they do not always include UV protection - you should ask your retailer if buying tinted lenses for use in the sun!
Photochromic lenses (commonly known as "Transitions" lenses)
Photochromic lenses automatically tint in the sun, and automatically clear up indoors. This is due to a chemical reaction designed within the lens. They're a great option who like tinted glasses when outside, but don't want to have to change pairs each time they go outside.
There are a few caveats. First, they don't change instantly - it takes time, usually a minute or two for a reasonable change. This can make it difficult for someone going between a dark indoors and a bright outdoors. They also don't work well in a vehicle - the vehicle's glass blocks many of the sun's rays entering the car, so they may not tint to the degree that an actual pair of shades would, or may not tint very much at all. Finally, some can be temperature sensitive and tint more/less based on the temperature, although the difference tends to be minor.
"Transitions" is a name-brand version of the lenses, although you may hear it interchangably (similar to how people refer to all tissue as "Kleenex", even if it's not Kleenex-brand). Unlike Kleenex, some brands tend to be better than others, and Transitions is commonly known as the "best" brand for Photochromic lenses.
Photochromic lenses vary in price, but they tend to add a fair bit to the price tag of lenses (particularly the Transitions brand). Many retailers offer photochromic lenses when you buy glasses.
Polarized lenses filter light that comes at different horizontal angles. They're ideal for driving, or for being out on the water, since they block out or reduce a lot of the light that comes from various angles. They're helpful for outdoor use when you tend to look in a particular direction and don't want the glare of the sun bothering you.
A few retailers offer Polarized lenses, varying in price.
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