Film Back EI 220 Exclusive film back for 220 roll film provides improved film flatness, with 30 exposures available on 220 in the 6 x 4.5cm format. * Safe film back system: The ETRSi has a built-in dark slide safety system for complete protection when interchanging film backs. The film back is not detachablbe without first inserting the dark slide and the dark slide is not detachable from a film back once it has been taken off the main camera body. This safety interlock means that there is full protection against accidental exposures.