Diffraction Effects in Video Due to Aperture
It is often a concern, when one is trying for the highest possible quality in image-making, what the effects on diffraction and image sharpness will be from the use of small lens diaphragm openings. In video, this is a concern since image areas (and therefore lenses and physical aperture sizes) can be quite small, indicating that the smallest optimum f-stops for best sharpness should be quite wide compared with those for larger image formats. Opposing this is the fact that video is generally a relatively low-resolution medium, and this is likely to "swamp" the subtle effects of most diffraction. To test the effects of diffraction on video image sharpness, I used one of the sharpest-imaging Mini-DV cameras available, the Sony VX2000, to maximize the visible effects of diffraction. I shot both motion-video and "Memory Stick" stills (at maximum quality level), but I present only the still images here since the motion-video frame-grabs were noticeably less sharp, and they therefore tended to hide more the subtle sharpness differences. This last may tell you the results, but looking at the still frames for each aperture available (f1.6 through f11) confirms that there is very little visible decline in sharpness with using smaller apertures in the Mini-DV video format. This was not a surprise, since even at f32 (available on some Sony one-CCD Mini-DV camcorders), video images still looked sharp (though using f32 in 35mm still work guarantees sub-optimal sharpness in normal-size prints). To keep the gain at "0" and the shutter speeds used fairly constant (to keep these variables from affecting the results noticeably), the tests were done in bright light, with these built-in neutral-density filter settings applied:
- ND no.2 for f1.6, f2, and f2.8
- ND no.1 for f4 and f5.6
- ND off for f8 and f11
Two frames were shot at each aperture setting, with the better selected for each. The manual focus was set at infinity. The effects of illumination roll-off at wide stops is visible (this would be partially covered with cropping to the TV-safe area), as is the growing size of the sun-reflection diffraction "star" as the diaphragm closes.
For more on video image characteristics and common defects, go here.
All of this is copyrighted material (David Ruether, 2002), and may not be reproduced without permission. Permission is granted to copy this material (including any of the still photos) for personal use only.
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David Ruether (email@example.com)