A coincidence rangefinder has two objective lenses separated by a fixed baseline B.
Each views the target at a slightly different angle (parallax); an internal prism
overlays both half-images. The operator turns a knob that shifts one image until the
two halves coincide — the knob position is calibrated directly in distance.
theta = 2 * atan( (B/2) / D ) (parallax angle for true distance D)
D_est = (B/2) / tan(theta_knob/2) (range read off the knob)
error = D_est - D_true
- Baseline length — physical separation between the two objective lenses; longer baselines give finer angular resolution at long range.
- True target distance — the real, physical range to the target (unknown to the operator).
- Coincidence knob — the operator's estimate; when it matches the true distance the split-image laser lines converge exactly on the target.
- Scope magnification — optical zoom, shown by how tightly the reticle rings frame the target.
WWII and Cold War-era warships and artillery batteries used exactly this coincidence
(or stereoscopic) principle — mechanical rangefinders many metres long — before laser
rangefinders made the technique largely obsolete.