This picture shows you 11 gray rectangles within a 50% gray box. You should be able to see all of the rectangles except the middle one. The rectangles represent the steps from 45% to 55% gray.
Esta imaxe mostra 11 rectángulos grises dentro dun cadro 50% gris. Debería poder ver todos os rectángulos excepto o do medio. Os rectángulos representan saltos desde o 45% ao 50% de gris.