When you’re buying new glasses, shopping online can be an easy and often cheaper alternative to visiting a traditional eyeglass store. As it turns out, depending on your prescription, it can also mean that those glasses you buy might not be quite as useful.
The issue comes from the fact that you’re not actually there to try on those frames, so the person making them doesn’t know how you’re going to look through them, Wirecutter reports.
Specifically, online glasses companies make frames so that the vertical centre of vision (where your eyes are looking through the frame) is in the centre of the frame. An optician at a shop; however, will measure where your centre of vision is and instead put that centre of vision where it actually is.
Wirecutter tried out a ton of different online glasses companies. While the company Flex + Iris did include a marker in its at-home try-on kits to have you mark your centre of vision, none of the others did. Instead, they essentially guessed at it by putting it in the centre. For some frames, that will work great. For others, you might be looking more toward the top or the bottom of the frames than the centre, which means your glasses won’t be as effective.
It’s not just the glasses fault. We’re all built a little different, so the same frames will likely fit me a little different than they fit you, that means we’ll both have a different centre of vision.
Wirecutter notes that if you have a weak prescription, then this likely isn’t going to be a huge deal. According to Dr. Neil Pence, associate dean for clinical and patient care services at the Indiana University School of Optometry who spoke with the site, in general, your prescription will need to be from about a -4 or a +4 for it to be a problem, but then it can be a pretty big problem.
People with stronger prescriptions will also have thicker lenses in their glasses. That lens “centre” is where the glass will be thinner. When yours is off, you’ll be looking through thicker glass, or oddly through thick and thin parts, which can make things look distorted or you’ll feel like you’re cross-eyed.
Online sites can be great for finding deals, but if you end up with a pair of glasses you can’t really see through, then that saving might not be worth it.