# How lens works in eyeglasses for distance and near vision?

Consider someone who wears a Contact Lens that is -10.00 diopters in power (very powerful) for distance correction, but needs +2.00 diopters ADD reading glasses "over-the-top" of the contacts to be able to work on the computer all day long:

Could the same optical correction be achieved by just using -8.00 contact lens for working on the computer up close, and using a pair of -2.00 **diopter eyeglasses over-the-top for distance vision?

This isn't a medical question. This is a question of optics (with a real-world example) and if the above scenario would given an optically equivalent result. It seems obvious to me that it would, but I'm not expert in physics or optics.

==EDIT== **In the second instance marked above, I at first wrote "+2.00 diopters" but I actually intended to write "-2.00 diopters" making the total power of contact lens plus glasses about -10.00 diopters.

## 3 Answers

As you know (or may not know) a diopter is the reciprocal of the focal length in meters, with negative numbers indicating the focal point is in front of the lens, so a -10 diopter lens has a focal length of .1 meters in front of the lens. Also, the further away from the eye the lens sits, the greater the effect, and vice versa. As Ramesh Chandra Huika points out, contacts lenses sit directly on top of the cornea, so for them to produce the same effect, the powers has to be greater.

According to The Physics Handbook the eyeball has a focal length of approximately 1.7 cm; a contact lens sits directly on the cornea, so it is (for the sake of argument) 1.7 cm from the focal plane. If we add 12 mm for eyeglasses, that represents a 70.58% increase in distance from first objective lens (the eyeglasses) to focal plane (the retina). Unfortunately, I cannot tell you if the ratio is linear or follows the inverse square law, though I suspect it's linear, as inverse square would lead to some ridiculously large (for positive prescriptions) or small (for negative prescriptions) numbers .

So to answer your original question, no you cannot simply add or subtract a reading glass prescription from your contact lens prescription. You'd have to factor in the difference in focal lengths between contact lenses and eyeglasses.

No, the power is different. In first case both contact lenses and reading glasses are contact with eye but later case the contact lenses are contact with eye but eye glass is at distance of12 mm Approximately.

• except for a small difference, would the diopter power "almost" be the same? I'm figuring that it would be about 0.50 diopter difference (less than a diopter). Also, I wrote an error in my question, and didn't catch it until now. The edit is marked above. So... -8.00 contacts (+) -2.00glasses would equal about -10 diopters of power. AND Without glasses would be like having the +2.00 reading glasses on with -10.00 contacts for 20/20 distance vision. ACTUALLY, I DO HAVE HIGH POWER CONTACTS AND THE PRESCRIPTION IS 0.50 diopter less powerful for contacts. That would be the only difference? – user12711 Sep 2 '18 at 23:41

From actual experience, I know that a -10 diopter eyeglass prescription corresponds to approximately a -8.5 diopter contact lens prescription, because the contact lens is closer to the eye. This means that the prescriptions between contact lenses and eyeglasses are not exactly 1:1. If you "detune" your contact lenses by 2 diopters for reading purposes, the eyeglass prescription required to make up the difference would probably be closer to -3 diopters, but you would need to consult an eye doctor to properly match the eyeglasses to the contact lenses.

Having said that, a better solution would seem to call for a -10 diopter contact lens prescription, with gradient lens reading glasses. The top of the lens has no power, so you wouldn't need to remove the reading glasses to see far away. As your line of sight goes farther down the gradient lens, the power increases, so there will be a line of sight that allows you to read confortably.

One last comment: I have no doubt that Lasik can correct your power, so if you want to get rid of one pair of glasses, this is the way to go. Prices are as low as \$250/eye, but I definitely would NOT recommend going to the lowest bidder, for the obvious reason. And yes, I have had Lasik, so I have personal experience with this procedure as well.