For each type of lens described below, its most common focal length measurements in 35mm film camera lenses and 16mm film camera lenses are provided. 16mm film camera lenses are half the measurement of their 35mm film camera lens equivalent. (A 50mm lens for a 35mm camera has the same angle of view as does a 25mm lens for a 16mm camera.)
Normal (Medium) Lenses (focal length measurements at or close to 50mm/25mm) provide an angle of perspective close to or equal to that of the human eye with little to no image distortion.
Wide Lenses (15mm-35mm/8mm-16mm) have a wider angle of view and more depth of field than do normal lenses. A wide lens can sometimes distort an image. The wider the angle of a lens and the closer the subject is to that lens, the more such distortion can become apparent, especially around the edges of the frame (a subject in the foreground may become disproportionately larger than a subject in the background). Wide lenses are often used for Establishing Shots, but are particularly useful for handheld and tracking shots as well because of their increased depth of field which makes it easier to keep a moving subject in focus. Wide lenses can also hide camera shake.
Narrow Lenses (Telephoto or Long) (85mm-250mm+/50mm-135mm+) have a narrower angle of view than do normal lenses with generally less depth of field. This allows the director to use selective focus to control where in the frame the audience focuses their attention. Narrow lenses sometimes have a flat, two-dimensional quality about them, but nevertheless remain particularly useful for close-ups because the camera can remain at a far enough distance so that no camera noise will be picked up by the boom microphone.
Prime Lenses have only one specific focal length that cannot be adjusted. Using a prime lens often results in a higher quality image than using a zoom lens. Prime lenses are also generally "faster" than are zoom lenses (see 'Lens Speed' just below).
Zoom Lenses are most useful to adjust the lens size (focal length) quickly. They are especially convenient for ultra low budget productions because one zoom lens can serve as a wide, normal, and narrow lens all in one. The act of zooming, that is changing the magnification of the subject within a shot, is considered bad form by most contemporary cinematographers. (Zooming makes a poor substitute for a dolly shot.) Zoom lenses are generally slower than are prime lenses and can sometimes produce images of lower quality.
Focal Point (Point of Focus) is the point on which the lens is focused and determines where in the frame the audience will focus their attention. It is determined by the distance of the subject that the lens is focused on to the camera's film gate (where the film is exposed). Depending on the lens used, and the chosen t-stop setting, objects outside of the focal point can be blurred and made unidentifiable (selective focus), or they may be included in the range of focus of that particular shot.
When focusing on a subject, do not depend on your eye alone. Use a tape measure to take a measurement from the film gate (usually this point is marked on top of the camera) to the subject's eyes (be careful not to poke out your talent's eye measure to the side of the face). On the lens, measurements in feet will be marked on the focal adjustment. Make sure the distance is accurate. Your shots must be in perfect focus for there is no way to fix them in the lab if they are not.
Depth of Field
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