Focal Length Calculator

How to Use This Calculator

1. Enter object distance and image distance to find focal length via lens equation.
2. Or use From Diopters tab — enter lens power in diopters.
3. Use Camera FOV tab to find field of view for a given focal length and sensor size.
4. Professional mode adds 35mm equivalent and hyperfocal distance.

Formula

Example

Frequently Asked Questions

• How do I find focal length from object and image distances? ▼
  1/f = 1/dₒ + 1/dᵢ, so f = (dₒ × dᵢ)/(dₒ + dᵢ). Both distances are positive for a real image situation. This is the thin lens / mirror equation rearranged to solve for f.
• How do I convert diopters to focal length? ▼
  f (meters) = 1/D. So +2 diopters → f = 0.5 m = 50 cm. +10 D → f = 0.1 m = 10 cm. Negative diopters give a negative focal length (diverging lens).
• What is field of view (FOV) for a camera lens? ▼
  FOV (horizontal) = 2 × arctan(sensor_width / (2 × f)). A 50 mm lens on a 36 mm wide sensor gives H-FOV ≈ 39.6°. Wider lenses give larger FOV; telephoto lenses give smaller FOV.
• What is 35mm equivalent focal length? ▼
  Focal length × crop factor. APS-C crop factor ≈ 1.5–1.6, Micro 4/3 ≈ 2.0. So a 25 mm lens on a Micro 4/3 sensor ≈ 50 mm equivalent on full-frame. Allows comparison between sensor sizes.
• What is hyperfocal distance? ▼
  H = f²/(N × c), where f = focal length, N = aperture f-number, c = circle of confusion (≈ 0.029 mm for full frame). Focusing at H puts everything from H/2 to infinity acceptably sharp — maximum depth of field.

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