The Cokin Filter A345 Double Mask 2 allows you to expose the image in two parts, combining two shots into one photo. The effect is achieved by taking two shots each with one filter from the Double Mask set. One filter covers the centre of the image, the other the surrounding area. This double exposure can be done by taking the shot and not transporting the film any further or by exposing the entire roll of film and then rewinding to its starting position and taking the shot with the second filter. It is best to measure the exposure without the filter attached to the camera.