Calculator
Example data table
| Diopters (D) | Focus distance (m) | Focus distance (cm) | Focus distance (ft) |
|---|---|---|---|
| 0.50 | 2.0000 | 200.00 | 6.562 |
| 1.00 | 1.0000 | 100.00 | 3.281 |
| 2.00 | 0.5000 | 50.00 | 1.640 |
| 4.00 | 0.2500 | 25.00 | 0.820 |
| -1.50 | -0.6667 | -66.67 | -2.187 |
Negative results may appear for diverging lenses or virtual focus conventions.
Formula used
- Diopters: D = 1 / f, where f is focal length in meters.
- Focus distance: For a lens at infinity focus, distance ≈ f.
- Unit conversion: Convert your distance to meters first.
How to use this calculator
- Select the calculation mode that matches your input.
- Enter diopters or focus distance, then choose units.
- Pick output units, precision, and a sign style.
- Press Calculate to show results above the form.
- Use CSV or PDF buttons to save the result.
Notes and practical tips
- A diopter value of 0 means an extremely long focus distance.
- Positive diopters typically represent converging lenses.
- Negative diopters often represent diverging lenses and virtual focus.
- For camera lenses, focal length is often given in millimeters.
1) What a diopter measures
A diopter (D) is the inverse of focal length in meters. It expresses optical power: larger positive values bend light more strongly and bring rays to focus sooner. This calculator converts that power into an equivalent focus distance, which is useful when comparing lenses, magnifiers, or corrective optics.
2) Fast conversion rule
The core relationship is simple: focus distance (m) = 1 ÷ D. For example, 2.00 D corresponds to 0.50 m, while 0.50 D corresponds to 2.00 m. Because the equation is linear in 1/D, small diopter changes at low power can produce large distance changes.
3) Typical diopter values
Many everyday tasks map to familiar ranges. A +1.00 D lens focuses at about 1 m, +2.00 D at 0.5 m, and +4.00 D at 0.25 m. In eyewear, prescriptions often include fractions like 0.25 D steps, so precision controls in the calculator help show practical increments.
4) How sign conventions matter
Positive diopters commonly represent converging optics that form real images for distant objects. Negative diopters often represent diverging optics; the “focus distance” may be interpreted as a virtual focus location. If you only need a magnitude for comparison, switch the sign style to show absolute values.
5) Close-up photography and magnifiers
Add-on close-up filters are frequently labeled in diopters. A +3 filter suggests a working distance near 0.33 m (33 cm). Stacking filters approximately adds diopters, so a +2 and +1 together behave like +3. Use the unit options to compare meters, centimeters, or inches quickly.
6) From distance back to diopters
When you know a desired focus distance, the reverse conversion is D = 1 ÷ distance(m). A target of 40 cm is 0.40 m, giving 2.5 D. This mode is helpful for estimating what optical power matches a working distance in a lab setup, inspection station, or reading aid.
7) Units, rounding, and reporting
Convert input distances to meters first, then apply the formula. Showing results in mm can be convenient for camera lenses, while cm is convenient for near-work distances. Rounding to 3–4 decimals is usually enough for planning, but higher precision can help when documenting measurements.
8) Exporting results for records
The CSV export creates a quick log for spreadsheets, experiments, or equipment notes. The PDF export captures the result card, including the selected units and the explanatory note. This makes it easier to share a consistent calculation record with students, technicians, or teammates.
FAQs
1) What is the formula behind this calculator?
Diopters are defined as D = 1/f, where f is focal length in meters. Rearranging gives focus distance (m) = 1/D for the infinity-focus case.
2) Why does a diopter of zero show infinity?
D = 0 means 1/f = 0, so f becomes infinitely large. In practice, it represents negligible optical power and an extremely long focus distance.
3) Are negative diopters “wrong” results?
No. Negative diopters commonly represent diverging optics. A negative focus distance can indicate a virtual focus position under a specific sign convention.
4) How do I convert 40 cm to diopters?
Convert 40 cm to meters: 0.40 m. Then compute D = 1/0.40 = 2.5 D. Use the “Focus distance → Diopters” mode to do this automatically.
5) Can I stack close-up filters?
Yes, their powers approximately add. For example, +1 D plus +2 D behaves like +3 D, which corresponds to a focus distance near 0.33 m.
6) Which output unit should I choose?
Use meters for general optics, centimeters for near-work distances, and millimeters for camera-lens style reporting. The calculator still shows meters as a reference.
7) Does this replace a full lens equation?
No. This tool relates diopters to focal length and an equivalent distance at infinity focus. Real imaging also depends on object distance, lens position, and system design.