Binocular AFOV Calculator

Measure Apparent Field of View Using the Flashlight Projection Method

Calculate the true apparent field of view (AFOV) of your binoculars using a simple flashlight projection technique. Verify manufacturer specifications and ensure accurate optical performance measurements.

Measurement Calculator

mm
Diameter of the illuminated circle on the wall
mm
Distance from eyepiece to wall (minus eye relief)
Optical Formula
AFOV = 2 × tan¯¹ (B ÷ 2 ÷ C)
AFOV: Apparent Field of View (degrees)
B: Diameter of illuminated circle (mm)
C: Distance from eyepiece to wall (mm)

Measurement Setup

Binocular AFOV Setup 1
Setup Alignment
Binocular AFOV Setup 2
Measurement Technique
View Detailed Procedure

Measurement Instructions

1. Setup Preparation: Mount the binocular on a tripod near a white wall, ensuring the eyepieces face perpendicular to the wall surface. The wider the AFOV, the more critical perpendicular alignment becomes.
2. Focus Setting: Set the focus to infinity. Note that some binoculars have field stops that can cause AFOV to vary with focus position.
3. Flashlight Projection: Shine a flashlight into the objective lens from close proximity to prevent light spillage onto the wall. Test different positions to achieve the largest, clearest illuminated circle.
4. Circle Measurement: Measure the diameter of the illuminated circle on the wall. If you cannot see the full circle at once, wiggle the flashlight to reveal opposite edges and mark them for measurement.
5. Distance Measurement: Measure the distance from the wall to the eyepiece's exit pupil location. If eye relief is known (e.g., 18mm), measure to the eye lens and subtract the eye relief value.
Important Notes:
  • Some binoculars exhibit edge-of-field darkening due to small rear prism apertures
  • Measure to the farthest visible edge of the illuminated circle
  • Ensure the binocular is stable and properly aligned during measurement
  • Perform measurements in a darkened room for best results