Sharpness: Each lens forms an image of an object in front of the camera on the lens plane as precise and sharp, this is often called the focal plane. In practice, this comprises a narrow depth range called depth of focus. In classic film photography, a large area displays the depth of focus. Blurring occurs when the so-called "breathing" of the film in the camera occurs. These might be curves of the film before the lens window, shifts in the lens system when focusing (through the action of a collision of forces), or improper distance setting. During projection there is a constant change of pressure with the mechanism that holds the film before the picture window. Occasionally, breathing of the film copy may occur if it is not thoroughly dried. Therefore, controlling the sharpness needs to be continuously followed up during projection (manual or automatic).