Lens decisions
Sep 1, 2008 12:00 PM, By Larry Thorpe
Select the correct auto-focus system for HDTV field and studio lenses.
Unambiguous focus is the essence of optimized HD video. Maneuvering the lens-camera system, framing the sought-for imagery, altering focal length under director instruction and selecting the scene object (from within complex scenes) of sharpest focus are all multifaceted tasks that only the camera operator can precisely manage.
Within this multitasking challenge, a considerable number of factors conspire against the camera operator seeking to maintain sharp focus in an HD lens-camera system:
- Certain combinations of subject distance, lens focal length and lens aperture can further reduce depth of field.
- Fundamentally, HDTV has a shallower depth of field than SDTV.
- Defocusing sensitivity is much greater in an HDTV lens than its SDTV counterpart.
- Contemporary HDTV cameras use viewfinders that do not have HDTV resolution.
- The subject within a scene selected for sharpest focus may be in motion, posing unique tracking challenges.
The phase detection approach to auto-focus
Figure 1. The main HD field lens is represented by the single large element. A smaller bundle of the primary light rays is passed to a second dual lens system that focuses these on to two assemblies of line array sensors.
Click to enlarge
Parallax occurs in panoramic photography if the camera and lens are not rotated around the entrance pupil of the lens and there is visible overlap between two adjacent images. Accordingly, to avoid parallax complexities, a through-the-lens (TTL) optical sampling system was adopted, which entails splitting off some of the light bundle sent by the lens to the primary camera imagers. This is augmented by a secondary image-registration, phase detection system that is used in most high-end SLR digital still image camera systems. However, the phase detection system for the HDTV lens had to be significantly refined beyond the capabilities of the digital SLR still imaging cameras. For the contemporary 2/3in long-zoom 100:1 HDTV field lens with a focal length of up to 930mm, the equivalent focal length translated to the 35mm image format is 3700mm.
Secondary image registration phase detection
Precise focus of the primary image plane (the HD camera's imagers) coincides with an exact focus of the two secondary images on their respective line array sensors. (See Figure 1.) When in focus, a hypothetical point is in precise point focus on the HD camera's three imagers, and that focus point is also precisely positioned in the center of the two line array sensors. The physical distance between these two centers is a specific reference known to the calculating algorithm.
Figure 2. The relationships between the two directions of primary lens defocus and the associated alterations in physical distance between the two defocused secondary images.
Click to enlarge
Figure 2 outlines the action of the system for two distinct situations of primary lens defocus (Figures 2a and 2c) and shows one in focus (Figure 2b). Figure 2a shows the situation when the main lens focusing element is moved to the rear. The main image is clearly defocused on the camera's imagers and at the same time the two secondary images are defocused on their respective line array imagers.
What is of special note, however, is the fact that the latter two defocused images have also moved in position toward the outer extremities of the line arrays. The physical distance between them has increased to Dl. Knowing that distance has increased unambiguously informs the algorithm that the main lens element must be moved forward to begin the refocusing of the image. This known predetermination is a vital contribution to the overall speed of the system. Figure 2c depicts the shortening of the distance when the main lens focusing element is moved to the front.
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