Amsler grid usually help detecting defects in central 20 degrees of the visual field.

[2] In the test, the person looks with each eye separately at the small dot in the center of the grid.

Amsler grids are supplied by ophthalmologists, optometrists or from web sites, and may be used to test one's vision at home.

A color version with a blue and yellow grid is more sensitive and can be used to test for a wide variety of visual pathway abnormalities, including those associated with the retina, the optic nerve, and the pituitary gland.

In 1869, Jacob Hermann Knapp described scotoma and metamorphopsia in traumatic maculopathy with choroidal rupture using horizontal lines.