Camera Lens Effective Focal Length Calculator (EFL Calculator)

What is Effective Focal Length (EFL)?

The focal length of a lens is the distance from its principal plane to the sensor when the lens is focused at infinity. It sets how “zoomed-in” your camera is, much like resistor value sets current in a circuit. The focal length requirement for your camera is also dependent on the image sensor size.

Engineers often talk about effective focal length (EFL) because real lenses have multiple glass elements and principal planes that don’t sit at the mechanical mount. EFL gives you a single number that still predicts field of view (FoV) exactly. Think of it as the optical “equivalent resistance” of a lens stack.

Field of view shrinks as focal length grows and expands as the sensor gets larger. Horizontal, vertical, and diagonal FoV each use the matching sensor dimension. An online EFL calculator saves you from manual trigonometry: enter sensor width, desired FoV, and it solves for focal length in millimeters.

Most tools assume the lens is focused at infinity, which is valid for robotics and machine-vision ranges beyond a few metres. If you work at macro distances, refocus and recalculate because EFL shifts slightly with object distance.

Distortion Impacts the Calculation. Real-world design rarely stops at EFL. Wide-angle lenses introduce barrel distortion which changes the FoV and EFL calculation. Check datasheets for distortion plots and ensure BFL + shim thickness equals your sensor flange depth.When selecting a lens, start with your required FoV, plug it into the calculator, and pick the nearest catalog lens that covers your sensor format.