The Canon EC-1Ds Black Mask Focusing Screen Set consists of two screens, one delineating a square format and the other a 4x5/8x10 format. The screens are based on the standard focusing screen for the 1Ds (Ec-CIII), but have an opaque black outside the aspect ratio area.
Both ambient and flash metering is affected when using the Black Mask screens. Automatic exposure is only 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.
Compatible camera models include EOS 1Ds, & EOS 1Ds Mark II.