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Lens Focal Length Calculator

Estimate the lens focal length (mm) and 35mm equivalent you need from your subject distance, real size, and sensor width.

Input

Estimate the lens focal length needed to fill the frame width with your subject, based on the subject distance, its real size, and your sensor (film) width.

m
cm
Sensor (film) size

Result

Focal length needed (to fill the frame width)

64mm

35mm equivalentapprox. 64 mm

Focal length (actual)

64 mm

35mm equivalent

64 mm

Horizontal angle of view

31.6 °

Calculation details

Sensor (film)35mm full frame (36mm)
Actual focal length63.5 mm
35mm equivalent focal length63.5 mm
Horizontal angle of view31.6 °
Magnification (approx.)approx. 1 : 46.2

How it works

  • The focal length f is estimated as 'sensor width × subject distance ÷ subject real size'. This is a thin-lens approximation derived from the image similarity ratio W / H = f / s, giving the focal length needed to fit the subject across the full frame width.
  • The 35mm equivalent focal length is found by multiplying the actual focal length by the crop factor of the chosen sensor (its angle-of-view ratio relative to full frame). Full frame is 1×, APS-C is about 1.5×, and Micro Four Thirds is 2×.
  • The horizontal angle of view is calculated as '2 × arctan(sensor width ÷ 2f)'. The shorter the focal length, the wider the angle; the longer it is, the narrower (more telephoto) the view.
  • Enter the distance in meters and the subject size in centimeters. The sensor width is based on the long-side (horizontal) dimension.
  • Because this assumes a thin lens, it does not account for real lens design, distortion, or minimum focusing distance. Use it as a rough guide when choosing a lens.
  • Results are rounded to the nearest whole millimeter. In practice, pick the closest standard lens value (24mm, 35mm, 50mm, etc.) for easier use.