Push-pull processing is a way in which film is developed under special circumstances. Photographers may choose to manually change the shutter speed to adapt to lighting for clearer images. Essentially, the photographer rates the film at a higher (push) or lower (pull) ISO than is assigned to the film roll. Push-processing means the film gets less light than the ISO number would suggest (i.e. nighttime shots), and thus needs a longer exposure time. Pull-processing is used in particularly bright or sunny situations and requires less exposure time. In order for the film developers to know how to properly process the film, the photographer marks the film roll with a + for push-processing and a – for pull-processing. If the film was originally marked as ISO 400, a +1 on the film would mean it is developed at ISO 800, +2 would be ISO 1600 and vice versa for pull-processing. It is important to note that the entire roll of film must be developed the same way; a single frame or image cannot be push-processed and another pull-processed in the same roll.