There is a difference between the prescription of frames and the prescription of contact lenses, and the higher the prescription, the more pronounced the difference. Therefore, you cannot simply buy contact lenses at the prescription of your frames. There is a conversion formula between the prescription of contact lenses and the prescription of frame glasses: contact lens diopter = frame glasses diopter ÷ (1 – 0.012 × frame glasses diopter). 1 diopter is equivalent to the usual 100 degrees, myopic diopter is negative, farsighted diopter is positive. 0.012 is the distance between the frame glasses and the highest point of the black eye, in meters. According to this formula, below 400 degrees, the difference between the diopters of frame glasses and contact lenses is not much and can be basically ignored. For those greater than 400 degrees, you will need to add or subtract degrees. Contact lenses used to correct myopia are lower in prescription than frames. For example, if the optometry result is 450 degrees of myopia, the contact lenses need to be reduced by 25 degrees and only 425 degrees will be needed. Contact lenses for farsightedness, on the other hand, are higher than frames. For example, if the optometry result is 450 degrees of farsightedness, the contact lenses need to be increased by 25 degrees and must be fitted with 475 degrees. Since glasses are generally graded at 25 degrees, not all prescriptions can be converted to the exact corresponding prescription. If there is no corresponding prescription for a particular prescription, you can choose the closest slightly lower prescription. For example, a 900 degree myopia frame should be fitted with 812 degree myopia contact lenses, but since there are no 812 degree lenses, you should choose 800 degree lenses.