Vernier acuity

A subject's vernier (IPA: /ˈvɜːrnɪər/) acuity is the smallest visible offset between the stimuli that can be detected.

Because the disalignments are often much smaller than the diameter and spacing of retinal receptors, vernier acuity requires neural processing and "pooling" to detect it.

[2][3] At approximately three to twelve months old, it surpasses grating acuity in foveal vision in humans.

[4][5] Vernier acuity was first explained by Ewald Hering in 1899,[6] based on earlier data by Alfred Volkmann in 1863[7] and results by Ernst Anton Wülfing in 1892.

[8] Vernier acuity is resistant to defocus, motion, and luminance, but is subject to practice effects and changes in attention.

Animated vernier scale (“Nonius”): The lower rectangle is divided into 10 sections but is equal in length to 9 sections in the upper scale. It is used, e.g., in a vernier caliper .