At the age of three years

Then visual acuity is measured at a distance of three meters (ten feet), first binocularly (= both eyes used), then the right and the left eye monocularly, while the other eye is covered. A line test is most effective in detection of differences between the two eyes if the symbols are only briefly pointed to. If the finger or a pointer is left next to the symbol to be read, it helps the child to hold the fixation of an amblyopic eye on the symbol. Glasses with one lens covered often are best accepted by young children.