Optical Power Calculator (Diopters)
Calculate the focal length and optical power of a lens in diopters.
Enter object and image distances to find magnification and lens strength.
Optical power describes how strongly a lens bends light. It is measured in diopters (D), which are simply inverse meters.
Power = 1 / focal length (in meters)
A lens with a 0.5 m focal length has 2 diopters of power. A lens with a 25 cm focal length has 4 diopters.
Sign convention:
- Converging (convex) lens: positive focal length, positive power
- Diverging (concave) lens: negative focal length, negative power
Eye prescriptions use diopters directly. A −2.0 D prescription means a diverging lens correcting myopia (nearsightedness). A +1.5 D means a converging lens correcting hyperopia (farsightedness).
Thin lens equation:
1/f = 1/do + 1/di
Where do = object distance (positive if on the incoming light side), di = image distance (positive if on the outgoing side, meaning real image; negative = virtual image).
Magnification:
m = −di / do
Negative m means the image is inverted. |m| > 1 means the image is larger than the object.
Lenses in contact: when two thin lenses touch, the total power is just the sum:
P_total = P₁ + P₂
An optometrist combining a sphere and cylinder correction exploits this directly.
Enter focal length alone for basic power conversion, or add object and image distances for magnification.