Formats and Focal Lengths

There is confusion about what lenses work on what format cameras and what adaptations are possible. Besides the back focus issue discussed in Retrofocus and Telephoto Lenses there are important issues concerning image formation of a lens.

First a few definitions.

The FOCAL LENGTH of any lens determines the height of the image a lens produces of an object. Different cameras with different formats use more or less of that image. How much we use of that image is determined by the FORMAT, the camera, and the COVERING POWER of the lens.
  1. FORMAT is the size of the film camera aperture or a video recording chip.
  2. COVERING POWER of a lens is the size of the usable image circle a lens produces. Every lens has a circular COVERING POWER optimized to fill the format of whatever camera format the lens is designed for. Covering power considers acceptable illumination and resolution which can both fall off at the extremes of the area a lens covers. A lens for a 16-mm movie camera will not "cover" a 35-mm still camera format. A 35-mm still lens creates an image much bigger than a 16-mm film or video camera requires but is optimized for the larger 35-mm format, so image quality is sacrificed.
  3. ANGLE OF VIEW. There is a relationship between the format and the focal length that determines the angle of view. The larger the format, the larger the angle of view for a certain focal length.















    4.

  4. RESOLUTION and ILLUMINATION. Every lens is a compromise for the format that it is designed for.
    A 4 x 5 still camera lens is designed to cover a very large image circle but can do that only at a reduced resolution. If you use only the center of the image produced by that 4 x 5 lens on a 35-mm movie camera it would give poor results. A lens for a 35-mm still camera would have a much higher resolution than a 4 x 5 camera lens, but could only cover a small circle in the center of a 4 x 5 camera format
Note: In motion pictures APERTURE has two meanings: a. A lens iris opening. b. The size of the opening of the film gate where the film is exposed. In video aperture has some more meanings yet.

Here are some examples of angle of view a lens "sees" on different camera formats.

For a 50-mm focal length lens that always creates an image the same height.
Camera Format Angle of View Comment
Camcorder/1/3" chip 4 degrees Very Telephoto
16-mm movie 8.5 degrees Telephoto
35-mm movie 21 degrees Slightly longer than "Normal"
35-mm still 47 degrees "Normal"
2-1/4 x2-1/4 still 75 degrees Very wide angle
4 x 5 still Super fish eye Won't cover 4 x 5 format


Other Factors
  1. The IMAGE QUALITY the lens must produce vs. cost.
  2. The BACK FOCUS necessary between the rear lens element and imaging device.
    a. A SLR still camera needs room for the reflex mirror to flip up and down behind the lens in front of the film.
    b. A motion picture camera needs room for the reflex mirror/ shutter to spin between the lens and film gate.
    c. A 3-chip video camera needs room for the beam splitting prism block that separates the image into 3 colors on to 3 separate recording chips. There is an additional problem of getting the light arriving at the prism block parallel enough to provide a good image.
  3. The PHYSICAL SIZE and WEIGHT limitation for the camera the lens is used on.
  4. For zoom lenses F-stop changes while zooming can be compensated by the camera for video and automatic exposure still lenses.
  5. Cost. The size and professional level of the market often dictates the cost.


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