Brian Walsh wrote: [A comment on a nice summary of definitions and design issues relating to selection of CCD and CMOS arrays for imaging applications, found at http://www.optics-online.com/literature/CCDlens.htm]
and Anthony Farr commented: > Thanks, Brian, for going the extra yard to get an expert point of view > from within the optical industry (rather than yet another "I reckon that ... > blah, blah, blah"). [Snip....] > I fear that, within two months, someone who [missed all the recurring > threads] will enlighten us that all the "rumours" of digital wide-angle > problems are just fear mongering, and that it really makes no difference > whether it's film or electronics at the focal plane because the light > doesn't know what it's striking! Gawd spare me! > Read my lips: film is not the same as an imaging chip, it has some > similarities but it also has significant differences. Hi Brian, Anthony, et al., Sheesh, Anthony, you seem a little sensitive about all of this. ;-) The website Brian points out is a nice summary. (By the way, thanks, Brian!) Since I'm apparently one of the folks with a prior "I reckon that... blah, blah, blah" discourse on imaging optics and sensors, I feel compelled to reply. But don't take it personally -- it's really not meant that way. Would it help lend credence if I started my dreadfully long discourses with "I'm a Ph.D. laser spectroscopist with several years experience designing low-light level sensing and imaging systems -- blah, blah, blah...." instead of "Well shucks, I reckon, blah, blah, blah...."? I find that the former approach usually just pisses people off, and I'm afraid I might end up with The Duchess beating me about the head. I usually just try to let the elegant physics speak for itself. If I fail to convince you, then I just didn't explain it well enough. But I digress.... One important point Brian's website alludes to is the significance of that microlens/microfilter array. This is really the element that makes a commercially available doped silicon color imaging sensor array (CCD or CMOS) behave much differently than film. This is the element which imposes constraints on the numerical aperture, or field of view, of individual pixels. (Take away this filter/lenslet array, and the differences between "silver halide sensor arrays" and doped silicon sensor arrays disappear. I'll show you how this is true below....) I think we're in complete agreement on the fact that light falloff for film vs. filter/lenslet-equipped color CCDs is different. As well we should be, since it's a consequence of some pretty simple physics. Unfortunately, a lot of folks who happen to find bits of photographic information -- on this list or elsewhere -- may not have sufficient background in optics or electronics. They'll read something like "You can't use existing photographic lenses with CCD sensor arrays....". They'll then see something erroneous that is passed off as an "explanation"; that is, "CCD sensor arrays are only sensitive to light which strikes perpendicular to the array....", and they'll accept that without question. Of course, you and I know that if the rays were all perpendicular to the array, and were thus parallel rays, you would have perfectly collimated light and no image at all. But those without sufficient background in optics will not know this, and they'll erroneously conclude that existing photographic lenses cannot be used with CCD or CMOS sensors. Then someone will offer a "correction" and say the rays must be "nearly parallel". That of course implies that only lenses of rather long focal length can be used with CCD or CMOS arrays. Which of course is utter nonsense. And thus propagates what I've described as the rumors and the fear-mongering. If you'd like to see some real live images collected through short focal length, "conventional" photographic lenses and sensed by CCD arrays, see the following: http://www.galaxyimages.com/gallery.html http://www.galaxyimages.com/conewidefield.html http://www.galaxyimages.com/orionccd.html The first URL is the gallery. The second was taken with a 19-35mm fl Nikkor camera lens set at 25mmfl at f/4. The third was taken with a 19-35mm fl Nikkor camera lens set at 35mm fl at f/4. Convinced yet? (Some of the others were taken at much longer focal lengths.) Ah, but these were acquired with a monochrome CCD sensor array, you notice. That pesky lenslet/filter thingy is not present in this sensor! OK, so if you want a color sensor array, you have a few design options.... Option (1): Throw out all of your existing lens designs, and redesign from scratch. However, keep in mind the following constraints from geometric optics; that is, -- Photons travel in straight lines, and no lens design will make them travel through free space to the image plane in curves; -- Light falloff for spherical optics goes as cosine to the fourth power. Option (2): Keep all your time-tested lens designs, and save all the money you'd spend on new designs, new tooling, etc. Spend this savings on designing a better lenslet/filter array to incorporate on top of a monochrome CCD. I think I'd go with Option (2) if I were that worried about light falloff with extremely short focal-length lenses. Must go now. I tip a glass of your favorite beverage to those of you who are still awake at the end of such a looooong post. Cheers, 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 .
