Horizontal eccentricity refers to the horizontal axis, measured in degrees, along the visual field. The blind spot extends from an eccentricity d₁ to eccentricity d₂ in temporal direction from the fovea. The size of the blind spot can be calculated as

${\displaystyle \arctan \left({\frac {d_{2}-d_{1)){d_{1))}\right)-\arctan \left({\frac {d_{2}-d_{1)){d_{2))}\right).}$