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.
Best Answer
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.