Find focal length from any diopter value. Reverse the conversion with mm, cm, meters, inches. Download CSV or PDF for records and sharing easily.
| Diopters (D) | Focal Length (mm) | Lens Type |
|---|---|---|
| -4.00 | -250.00 | Diverging (minus) |
| -2.00 | -500.00 | Diverging (minus) |
| -1.00 | -1000.00 | Diverging (minus) |
| 0.50 | 2000.00 | Converging (plus) |
| 1.00 | 1000.00 | Converging (plus) |
| 2.50 | 400.00 | Converging (plus) |
| 4.00 | 250.00 | Converging (plus) |
The diopter (D) is optical power. It equals the inverse of focal length in meters: D = 1 / f(m).
A diopter (D) measures optical power as “per meter.” It tells how strongly a lens bends light. Focal length is the distance where parallel rays meet (or appear to diverge from). Because power increases as focal length shortens, diopters and focal length are inversely related.
The calculator uses D = 1 / f(m). Converting meters to millimeters gives f(mm) = 1000 / D. For example, +1.00 D equals 1000 mm, while +2.50 D equals 400 mm.
Everyday eyewear often falls near ±0.25 D steps. Mild correction may be around ±0.50 to ±2.00 D (focal lengths roughly 2000 to 500 mm). Stronger lenses can reach ±6.00 D or more (about 167 mm).
Positive diopters represent converging lenses; negative diopters represent diverging lenses. The sign is preserved in focal length: -2.00 D gives -500 mm. That negative sign is useful for consistent math in optics, even though “distance” feels positive in everyday talk.
Lens calculations are naturally defined in meters, but many specifications use millimeters. This calculator converts through meters to avoid mistakes: 25.4 mm equals 1 inch. You can input a focal length in cm, m, or inches, then standardize the output.
Small diopter changes can cause noticeable focal changes when power is weak. For instance, 0.25 D corresponds to 4000 mm, and 0.50 D to 2000 mm. The tool blocks zero diopters because that would imply infinite focal length.
Try these checks: +4.00 D → 250 mm, +2.00 D → 500 mm, +1.00 D → 1000 mm. Notice the pattern: doubling diopters halves focal length. Reverse mode confirms consistency by converting the focal length back into diopters.
Close-up camera filters are commonly labeled +1, +2, or +4 D, mapping to 1000, 500, and 250 mm. Magnifiers, lab optics, and quick classroom demos also use diopters for fast comparisons across different lenses and setups.
It means a focal length of 1 meter. In millimeters, that is 1000 mm. The relationship is inverse: higher diopters mean shorter focal lengths.
Negative power indicates a diverging lens. Using a negative focal length keeps the optics sign convention consistent in formulas, ray tracing, and combining multiple lenses.
Yes. If focal length is in millimeters, use D = 1000 / f(mm). For example, 500 mm equals 2.00 D, and 250 mm equals 4.00 D.
Zero diopters would mean infinite focal length, like a perfectly flat window that does not focus parallel rays. The calculator blocks zero to prevent division-by-zero errors.
The math is exact for the definition of a diopter. Real lenses may differ slightly due to thickness, refractive index, and design, but the conversion is the correct starting point.
Millimeters are convenient for cameras and lab lenses. Meters are clearer for low powers such as 0.25 D. Inches help when working with imperial hardware or printed measurements.
Because focal length changes rapidly at low power. Going from 0.25 D to 0.50 D halves focal length from 4000 mm to 2000 mm, which is a large shift in focusing behavior.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.