ABSTRACT
Except for certain minor differences, the principles of optics and the use of lenses are the same for film and video. The focal length of the lens is the distance between the optical center of the lens and the image sensor when the subject is in focus; it is usually stated in millimeters such as 18 mm, 50 mm, or 100 mm. The advantage of extension tubes or bellows is that they do not alter the optics at all, so there is no degradation of the image. Extension tubes are rings that hold the lens farther away from the film plane than it normally sits, thus reducing the minimum focus distance. Macro lenses are actually specially designed optics, optimized for close-up work. Focus is a much misunderstood aspect of filmmaking. The viewing audience will tend to focus their attention on the part of the image that is in focus.
