This post originally appeared onDealNews.

Here are six things to know when buying glasses online.

Your Prescription Status

Youll need a current prescription to order glasses online.

Check to see if yours has an expiration date.

If it doesnt, contact your eye doctor to ask.

If your current prescription has expired, youll need to schedule an eye exam first.

These are the three measurements used for frame sizing:

Lens width: Exactly what it sounds like.

Bridge width: The size of the piece that fits over your nose.

Temple arm length: The length of the arms that extend back toward your ears.

Its a crucial measurement because your lenses need to be centered on your pupils.

If they arent, the glasses will likely cause eyestrain and make it hard for you to focus.

Your pupillary distance may be written on your prescription.

There may be a fee for measuring, but it isnt likely to break the bank.

Theyre an ideal choice for kids or for general wear when playing sports or doing other outdoor activities.

High-Index Plastic Lenses

As the name implies, these lenses have a higher index of refraction.

The higher the refractive index of these lenses, the thinner they are.

The biggest brand in photochromic lenses is Transitions.

For lens coatings, someif not allof the coatings were about to list may be included with your lenses.

However, that doesnt mean you wont be offered better (and more expensive) versions of these coatings.

Are higher-quality coatings worth it?

Maybe… and maybe not.

The latter can be especially good if you work with computers frequently.

Scratch-resistant coatingis just what you think it is.

Many lenses are capable of blocking 100% of UV light without any additional coating, though.

Anti-fog coatingcan keep your glasses from fogging up when you come in from the cold.

Check before you buy!

Is the Retailer Trustworthy?

A web search for buy glasses online brings up dozens of stores, but which ones right for you?

Also check the stores return policy and warranty.

Some retailers wont allow you to return prescription lenses, while others offer generous return windows.

Image byAndrew Jassovia Unsplash.