The charts standardized the test task so that size is the only significant variable from one row to the next. There are 5 letters per row, between-letter and between-row spacings equal to letter size, size progresses by a constant ratio (0.1 log unit = 1,26x), and letter difficulties are balanced for each row.
The difference in the number of letters read on the high and low contrast charts provides a measure of the slope of the Contrast Sensitivity Function.
The charts come as a set of two panels (21x24inches, 53x60cm) each with a high contrast chart one side and a low contrast chart (18% Weber or 10% Michelson) on the other. The optotype set is the family of ten British Standard (1968) 5x4 non-serifed letters. There are 14 rows with a range of sizes from 38 to 1.9 M-units. Sizes are given in Snellen units (in both feet and metric units), as well as visual acuity values in LogMAR and VAR (Visual Acuity Rating) for a 6 meter viewing distance. Any other viewing distance may be used as well. A scale printed on each chart provides the score adjustment for different test distances. With the LogMAR and VAR scales, equal credit is given for each additional letter read (each extra letter earns -0.02 with LogMAR , or 1 point with VAR)