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|Focus/zoom/aperture combinations for critical sharpness||search|
Anamorphic lens adapters are essentially 0.66x wideangle adapters in the horizontal direction only. Because of this, they change the lens's focal point (and nodal point, too) in the horizontal direction only; the lens focuses differently horizontally and vertically!
While an adapter can be designed to minimize this effect, the optimization only holds for a given focal distance and zoom setting: all other combinations are compromises. To compensate, and preserve critical sharpness, one must stop down so that the increased depth of field allows the same point in space to be focused both horizontally and vertically.
|NG = never sharp; - = can't focus DVX100 that closely. © 2004 Adam J. Wilt|
The table lists maximum apertures for different zoom/focus settings for the AG-LA7200G adapter on the AG-DVX100 camera. The camera's zoom/focus readouts make this possible. I set the camera on progressive scan, thin vertical detail for maximum vertical resolution, and shot a star chart and resolution test chart for maximum detail discrimination. I monitored via Y/C on a Sony PVM-1344Q rated at 600 TVL/ph resolution, and listed the widest aperture for each focus/zoom combination for which I didn't detect any sharpness degradation. These numbers may be somewhat on the conservative side. Your mileage (and sharpness) may vary.
Note that the picture will look adequately sharp in the viewfinder or the flip-out LCD at much wider apertures; these finders lack the resolution for critical focusing. In particular, it's not possible to detect vertical sharpness adequately with them as they only offer 240 vertical pixels: half the frame resolution.
Zoom settings in mm and focus settings in feet & inches are approximate. Focus points split the difference between best horizontal and best vertical sharpness. Settings for a focus point of 90 were determined by focusing on distant foliage; I could not locate the star chart far enough away.
The practical near focus limit is about 14: the front glass of the adapter, and the dust on it. Infinity focus appears to be at a setting of 92 or 93. The DVX100A allows nearer focusing in telephoto (at full tele, the near focus is 37 instead of 50), so anyone who can supply me with the numbers to fill in the blanks is welcome to do so!
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Last updated 2004.05.15