Optical power (also referred to as dioptric power, refractive power, focusing power, or convergence power) is the degree to which a lens, mirror, or other optical system converges or diverges light. It is equal to the reciprocal of the focal length of the device: P = 1/f. High optical power corresponds to short focal length. The SI unit for optical power is the inverse metre (m-1), which is commonly called the dioptre.
the "power of lens" is actually the reflective power,and can't be measured until you go to the eye dr.the test the dr will give you to come up with your eye glass prescription is an objective test,meaning,you are telling the dr which lens you see clearer out of,1 or 2 and so on.if you already wear glasses currently,the power can be read on a device called a lensometer.only a trained pro can read the "power".
Power of lens = 1 / (Focal length in meters)