"Raimo Korhonen" <[EMAIL PROTECTED]> wrote: >Maybe this could be avoided by the use of some kind of optics in front of the chip?
The legendary Bill Peifer addressed this many moons ago: "Peifer, William [OCDUS]" <[EMAIL PROTECTED]> wrote: >All this talk about "analog" vs. "digital" lenses has got me wondering a >bit. I'm curious where this whole idea of CCD sensors requiring (or >preferring) perpendicular rays originated. I'm pretty convinced that it >must have originated because somewhere along the line, something got taken >out of context, and a fundamentally incorrect idea grew from there. From >the standpoint of the underlying physics, Tom is absolutely right -- the >purpose of a lens is to bring an image to critical focus at the focal plane, >and the nature of the sensor (film, CCD, CMOS, or other) isn't particularly >relevant. After all, if all the light rays strike the sensor >perpendicularly, then they are necessarily parallel and thus cannot form an >image at the focal plane! > >I suspect that this perpendicular-ray story -- dare I say "legend"? -- may >have originated from a misinterpretation of the characteristic behavior of >CCD sensors. We all know that in single-chip color CCD sensors, some of the >pixels are sensitive to red, others to green, and still others to blue. For >the case of color cameras with single CCD sensors, color sensitivity is >imparted to a particular pixel by incorporating a microscopic optic -- a >lenslet and filter -- in front of that pixel, which I believe is >accomplished as part of the manufacturing process for the sensor chip. I >can imagine that the numerical aperture of this microscopic optic may not be >terribly large, and it might very well constrain the field of view of its >corresponding pixel. Maybe someone that knows more about chip fab can >comment on this. Anyway, although each individual pixel may very well be >"looking" through an optic with small numerical aperture, it's only >"looking" a very short distance (microns? tenths of microns?) to the >illuminated spot on the focal plane directly in front of it. In fact, this >is precisely what you want. If each pixel had a more "wide-angle" view, it >would not only register the intensity of light directly in front of it, but >it would also register the intensity of light from a immediately adjacent >pixels (perhaps pixels intended to sense a different color), resulting in a >spatially and chromatically degraded image. The characteristics of the >macroscopic, "analog" lens mounted onto the front of the camera -- focal >length, f-number, etc. -- isn't particularly relevant, except that a faster >"analog" lens will make each pixel-size spot of light at the focal plane >correspondingly brighter. > >Jaume's original question about spectral characteristics of particular >lenses and lens coatings is interesting as well. The general strategy in >designing the ~lens~ is, among other things, to reduce chromatic aberration; >that is, to get red, green, and blue rays from a single object point to >focus at a single point on the same focal plane. I think lens ~coatings~ >are generally optimized to match the response of the human eye, rather than >the film emulsion. (Likewise, most film emulsions -- excluding infrared, of >course -- are designed to match the human eye.) I believe that the general >strategy in designing antireflection coatings (like SMC) is to minimize the >reflective loss of green light, since green is the color our eyes are most >sensitive to. This doesn't mean that the coated lens passes primarily green >light; rather, it means that for the 1% or 2% of light that would otherwise >be lost at each air-glass interface of an uncoated lens element, the lens >designers try to "rescue" the green component by applying a green-optimized >antireflection coating. CCDs are more sensitive to the red end of the >spectrum than the human eye. You might imagine that in order to maximize >the signal level at the focal plane of the CCD, a lens designer might >consider using antireflection coatings optimized for passing red light. >However, this would yield an image with what we would perceive as a highly >perturbed color balance. In fact, for consumer imaging applications, >designers use filters that ~decrease~ the intensity of far red and near >infrared light impinging on the sensor. Thus, I can't imagine that consumer >digital camera designers would go to the expense of new lens designs, or >bodies specific for old vs. new lenses. (Although that would certainly be >an interesting marketing gimmick....) > >Just as a final aside, I'll mention a pet peeve of mine. It seems that in >many discussions, we refer to film-based and CCD-based imaging as "analog" >and "digital". This is really an artificial distinction. CCDs, after all, >~are~ analog sensors, and the readout electronics for CCDs are analog >circuits. The only thing that makes "digital" cameras digital is the way >the analog signal array is stored after being read off the CCD sensor. A >minor point, but a pet peeve nonetheless. > >Bill Peifer >Rochester, NY >- -- Mark Roberts www.robertstech.com - This message is from the Pentax-Discuss Mail List. 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