JVC NAB-2007 manual SD lenses on HD camcorders?

JVC Professional APRIL 2007 NAB-2007 The ProHD Report

SD lenses on HD camcorders?

TV stations generally own large quantities of SD lenses, some even purchased recently. However, most of these lenses are ½-inch and 2/3-inch, and thus require lens adaptors to fit on the 1/3-inch camcorders. JVC does offer both ½-inch and 2/3- inch lens adaptors for the ProHD camcorders. There are two primary problems associated with using SD lenses on HD camcorders:

Chromatic aberration in the lens is (simplistically) that a beam of light containing different colors (as any light ray is made up of the primary colors) diffract differently through a lens element, like light is split into the primary colors by a prism. In an extreme case example, a pixel-size light ray (containing red, green and blue components) going through a lens element is diffracted into three beams of red, green and blue, and thus being “out of registration” before entering the camera front end. With HD being 6x the area resolution of SD, chromatic aberration (CA) is much more challenging in HD, and the lens manufacturers take great care in the design and the manufacture of HD lenses to reduce the CA to a minimum. SD lens design were of course performed to a SD standard with respect to CA, therefore the official recommendation is not to use SD lenses on HD camcorders. CA is particularly observable at object edges in the image, with perhaps a spurious color edge being visible in contrasted transition from light to dark or dark to light, due to the “out of registration” color separated pixels.

Longitudinal chromatic aberration happens as the light beams travel through the lens, and, not surprisingly, CA gets worse with longer focal lengths (at telephoto settings). Lateral chromatic aberration is measured from lens center out toward the edges, as it is impossible to maintain lens center CA performance as one approaches the lens edge. In the question of using SD lenses on HD ENG camcorders, these CA problems may not be sufficiently adverse to prevent the use of SD lenses, as most ENG stand-up remote reporting only uses the middle of the 16:9 screen for the talent and uses a wide lens setting rather than telephoto.

Lens adaptor multiplier effect. The ProHD camcorders are 1/3-inch imager where optimum matched lenses are also 1/3-inch. The use of lens adaptors of ½-inch-to- 1/3-inch and 2/3-inch-to-1/3-inch produces the effect of “multiplying” the 1/3-inch focal length (reducing the angle of view). In the ProHD camcorders, a 1/3-inch lens with a focal length of 5mm produces a horizontal angle of view of 52 degrees (a relatively wide angle).

Using a ½-inch lens (with a native min focal length of 5mm) with the adaptor increases the focal length by a factor of 1.43 to 7mm, producing a horizontal angle of view of approx. 37 degrees, which may be acceptable in HD ENG.

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