Lens Power Calculator
Find lens power in diopters from focal length.
Lens power (used for eyeglasses) is the reciprocal of focal length.
How the Math Works
The Lens Power Calculator uses the fundamental optical formula P = 1/f, where power (P) is measured in diopters and focal length (f) is measured in meters. This relationship shows that lens power is inversely proportional to focal length: as the focal length decreases (the lens bends light more sharply), the power increases. For example, a lens with a 0.05-meter focal length has a power of 20 diopters (1 ÷ 0.05 = 20). The formula works because diopters quantify how strongly a lens converges or diverges light, making it the standard unit for lens prescription worldwide.
Practical Applications
To use this calculator practically, first measure or obtain the focal length of your lens in meters. If you have the focal length in centimeters or millimeters, convert it by dividing by 100 (for cm) or 1,000 (for mm). Simply enter the focal length value and divide 1 by that number to find the lens power in diopters. Optical technicians use this calculation when manufacturing corrective lenses, while photographers may use it to understand the magnification and field of view characteristics of their camera lenses during equipment selection.
Day-to-Day Use
This calculation directly impacts your daily life through eyeglasses and contact lenses. When you receive a new prescription, understanding that your eyes need 2.5 diopters of correction helps explain why certain prescriptions appear stronger than others. It also enables you to compare lens strengths when shopping for eyewear, understand why reading glasses have positive power while some specialty glasses have negative power, and appreciate why camera lenses with longer focal lengths (smaller diopter values) provide zoomed-in views while wide-angle lenses (higher diopter values) capture broader scenes.
Worked example
0.5 m → +2 diopters.
FAQ
Glasses prescription?
It's given directly in diopters (e.g. −2.00).