This special effect filter allows you to expose each half of one piece of film separately. One half of this attachment is completely black and blocks all light transmission while the other half is transparent. With the first exposure one half of the image remains unexposed. Then by rotating the attachment in its holder 180 degrees and using the multiple exposure feature on your camera a second exposure is made on the unexposed half of the film. This filter measures 85 x 85mm and fits the Cokin P series holder. NOTE: This filter can only be used with cameras that allow for multiple exposures.