The Canon EC-1D Black Mask Focusing Screen Set consists of two screens, one delineating a square format and the other a 4x5/8x10 format. The screens have an opaque black outside the aspect ratio area. Ambient and flash metering are affected when using the Black Mask screens. Automatic exposure is usable in partial, spot, or multi-spot metering patterns. When using E-TTL or E-TTL II autoflash with Canon EX Speedlites, only FEL (Flash Exposure Lock) together with partial or spot metering is recommended.