0 SDTV Lens on HDTV Camera: To Be or Not to Be?

The essential distinction between SDTV and HDTV is bound up in the core issue of the term “high definition”. It implies that HDTV is distinguished from traditional video primarily by more “definition” – thus producing much higher picture sharpness. Thus, any discourse on mixing SDTV products with HDTV products needs to be closely examined from the viewpoint of its impact on the “high definition” aspect of the final imagery. Unlike the digital cameras and recorders that make up contemporary digital imaging systems, the lens is a totally analog technology. It is a very physical technology, in the fullest sense of the word. The lens is also dynamic— in terms of the substantial degree of control it can exercise over the object image that it presents to the camera image sensors.

Those variations in light level, focus, and focal range offered by the lens come with some technical penalties. As this paper is intended to examine “definition” the focus will be on the behavior of lens resolution. It is useful to establish some metrics in optical terms, for picture “definition”. Audio and video systems (such as a television camera) are described by considerations of bandwidth and the specific system responses over the frequency ranges encompassed within their respective bandwidths. A similar approach can describe the resolution performance of a lens.

OPTICAL BANDWIDTH
Lens Contrast and Resolution are inextricably intertwined. A series of closely spaced alternating black and white lines are visually distinguished by their relative contrast to each other. As their thickness and spacing are progressively reduced our human visual system is tasked to distinguish between these alternating lines. At some point we fail to do so and they blur into a gray patch. The same thing occurs as the test chart object scene passes through a lens. As the alternating lines increase in spatial frequency their optical representation by the lens will exhibit a progressive roll-off as simplistically illustrated in

Figure 1. In other words, the contrast reproduction capability of the lens is modulated as a function of the fineness of detail of the alternating black and white lines. This particular representation of the lens output is technically termed the Modulation Transfer Function – or MTF. The horizontal axis represents the spatial frequency (increasingly fine detail from left to right) in Line-pairs per millimeter (Lp/mm). The vertical axis is the contrast (amplitude of black to white) of the optical image output of the lens.

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