Both require sizing information, for most the amount of correction is the same. Contacts contour to your eyes. Glasses have center the correction over the center of each eye, not in the center of the lense.
There are some cases where the actual correction might be different. It’s much easier to handle astigmatism in glasses than contacts so the glasses prescription might include that where the contacts don’t. You might have bifocal glasses and monofocal contacts, especially if you’re older. Most are going to be the same, though.
Contacts have to fit your actual eyeball, so in addition to the correction values, you need to get fitted to the contacts that fit your eye curvature properly. So your contact lens prescription will likely be for a specific brand/”model” of contacts, plus the correction, as well as any specifications of astigmatism. Additionally, the correction values might change slightly because the lenses are right in your eyeball instead of an inch or so in front of your eye.
Glasses sit further away from your eye, so the eyeglass power needs to be higher. Practically speaking this only comes into play if your power is greater than +/- 4 though.
The contacts prescription includes a measurements of your eyeball (curve, diameter) for a better fit. Sometimes the optometrist can also prescribe specific brands depending on what suits you.
Contacts don’t come in all power increments, and sometimes this differs by brand. So the optometrist may need to round the prescription up or down.
If you have astigmatism then you’ll need a prescription for a different kind of contacts. There are a lot of other conditions where the two greatly diverge.
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It has to do with how far the lenses are from your eye
Both require sizing information, for most the amount of correction is the same. Contacts contour to your eyes. Glasses have center the correction over the center of each eye, not in the center of the lense.
There are some cases where the actual correction might be different. It’s much easier to handle astigmatism in glasses than contacts so the glasses prescription might include that where the contacts don’t. You might have bifocal glasses and monofocal contacts, especially if you’re older. Most are going to be the same, though.
Contacts have to fit your actual eyeball, so in addition to the correction values, you need to get fitted to the contacts that fit your eye curvature properly. So your contact lens prescription will likely be for a specific brand/”model” of contacts, plus the correction, as well as any specifications of astigmatism. Additionally, the correction values might change slightly because the lenses are right in your eyeball instead of an inch or so in front of your eye.
Another question. Why is the contacts prescription only good for a year while the glasses prescription is good for 2 years?
The two can be different.