This is generally true except for the following: > 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
If we were talking about old single coated lenses, I would agree. The single coated compensation is (essentially) a 1/4 wave (and therefore frequency sensitive) transformer matching the impedance of air to the impedance of the glass and would be most sensitive to approximately the center of the spectrum which is green. It will also act as a filter sensitive to green light. Now, the response is rather broad so that all visible frequencies benefit, but green does benefit better. This will produce a slight green cast, but this is not really noticeable unless you have a lot of coated surfaces. With each coated surface, filter is stacked on filter and the response to green becomes sharper. Different surfaces could have different coating thickness and/or different indices of refraction to equalize passage of blue and red giving a lens that overall would be returned to high color fidelity, but some transmission loss would result and there would also be flair - better than no coating, but flair none the less. A better approach is to use a broad band (flat response) transformer/filter at each surface. This will produce better overall light transmission at each surface and (as a result) less flair. This is what multicoating does. It produces a *broadband*, impedance matching transformer for less loss and less flair. Regards, Bob... -------------------------------------------- "Do not suppose that abuses are eliminated by destroying the object which is abused. Men can go wrong with wine and women. Shall we then prohibit and abolish women?" -Martin Luther ----- Original Message ----- From: "Mark Roberts" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Friday, August 08, 2003 7:50 PM Subject: Re: Full frame DSLR and Film-Camera-Based Lenses > Richard Chu <[EMAIL PROTECTED]> wrote: > > >Olympus' brochures claim that the film-camera-based > >lenses would not provide good results with the CCD > >sensor because the light rays, especially at the > >edges, do not properly strike the sensor at an right > >angle. Does anyone know if images taken with Canon's > >full frame DSLR have any issues at the edges? If yes, > >if Pentax does come out with a full frame DSLR, > >wouldn't we need to buy new designed lenses to get the > >best results? If Olympus' claims are true, we should > >not count on using the existing lenses on a full frame > >DSLR if we want to match the results of the film > >cameras. The smaller CCD sensor in the *ist D is a > >compromise but it allows us to continue to use our > >existing lenses with good results. > > Here's a post from one-time PDML member Bill Peifer on this subject. > (He's PhD who works in optics in Rochester): > > 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. > > -- > Mark Roberts > Photography and writing > www.robertstech.com > > >
