Not just that. The common type of CCD/CMOS array currently being used is front illuminated. That means the part that illuminates the pixel is in front. This means the light getting to it must pass through and around this part of the array. As a result I'm pretty sure in the case of front illumination the angle of incidence (angle it strikes the array) is very critical to minimize distortion from the hardware in front. Also this hardware forms a layer in front of the actual pixel array itself which any dust will get on. So blowing it off with air will not damage the array itself as it is under this layer. The reason they are transitioning to rear illumination is that the definition of each pixel improves without this front hardware distorting the light path. Of course to work effectively in rear illumination the space between the illumination and the pixels themselves must be even smaller than in front illumination. As a result rear illumination arrays have to be polished to a rear thickness of only 10 microns, making them more delicate right now. I'm also of the opinion that due to the differences between how the film lies and how the array lies that lenses for digital cameras also needs a flatter field than is necessary with film. Because the only cameras where the film lies absolutely flat is ones with vacuum backs on them. So there is likely a little more leeway in field curvature in film cameras than in digital ones where the array is absolutely flat to some fraction of a wave of sodium light. Kent Gittings
-----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Peifer, William [OCDUS] Sent: Monday, October 29, 2001 11:42 AM To: '[EMAIL PROTECTED]' Subject: RE: 'analogical' lenses coating and CCD, not fully compatible? On the subject of "analog" lenses, Tom C. (aimcompute) wrote: > If any lens of sufficient quality attached to the camera body achieves > a critical focus on the focal plane, and the image transmitted to the > focal plane covers the entire sensor area, be it CCD or film, that's all > that matters.... and Rob Studdert replied: > Maybe but there is possibly an angle of incidence factor ie film is > not as sensitive to the angle of the light hitting its surface whereas > the CCD cells really function optimally when hit by perpendicular > rays.... Hi Tom, Rob, et al., 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 - This message is from the Pentax-Discuss Mail List. To unsubscribe, go to http://www.pdml.net and follow the directions. Don't forget to visit the Pentax Users' Gallery at http://pug.komkon.org . ********************************************************************** This email and any files transmitted with it are confidential and intended solely for the use of the individual or entity to whom they are addressed. If you have received this email in error please notify the system manager. This footnote also confirms that this email message has been swept by MIMEsweeper for the presence of computer viruses. www.mimesweeper.com ********************************************************************** - This message is from the Pentax-Discuss Mail List. To unsubscribe, go to http://www.pdml.net and follow the directions. Don't forget to visit the Pentax Users' Gallery at http://pug.komkon.org .
