ZALA Single 180 Degree Split Image Focusing focus Screen For Canon EOS 550D 600D DSLR Does not affect your camera's AF ability.Easy to focus, especially with objects vertical lines Does not affect the accuracy of focusing. Does not affect the brightness of the screen. The focusing screen is a flat matte-finish translucent plastic with a split-circle in the middle, surrounded by a ring field of micro-prism. When the image is correctly focused, the circle becomes full and the line between will be unbroken. Otherwise, the line separates the circle into two halves.