Mark wrote:
[Questions on the origin of advice not to use "conventional" lenses with
"digital" imaging sensors....]
Hi Mark,
It's poor advice, not technically rigorous, and just plain wrong in some
instances. In fact, the current issue of Sky & Telescope has an article
about a fellow who now uses -- and sells -- adapters to mount Nikon lenses
on CCD camera backs. (These camera backs are cooled monochrome CCD
detectors made by Santa Barbara Imaging Group and are marketed to the
amateur and professional astroimaging community.) You'll have to buy the
magazine to read the article but see Table of Contents ("Wide-Field Imaging
with CCD Cameras" by Steve Mandel) at the following URL:
http://www.skypub.com/skytel/contents/current.html
The problem -- when one actually does exist -- is that color image sensors
have a filter array mounted in front of them. Thus, some pixels get red
photons, some get green photons, and some get blue photons. Because of
pixel-filter spacing, optical masking, etc., a given pixel-mask-filter
element may not have a terribly good numerical aperture (i.e., a limited
angle of view). Thus, light falloff at the edges of the image will be more
severe for this particular design of sensor array than for a monochrome
sensor array (without filters, masks, etc). I don't believe you can change
the camera's lens design to compensate for this. I would imagine that the
most practical solution is to incorporate a lenslet array directly in front
of -- or incorporated into -- the color filter array. I'd also guess that
this is more expensive to implement, and would drive up the cost of any
camera in which it was used.
I believe that the cameras you mention don't use full-frame sensors. They
probably still suffer from worse light falloff at the edges than film, but
with a smaller sensor array, the final picture ~looks~ like it doesn't
suffer as badly from light falloff.
Hope this helps.
Bill Peifer
Rochester, NY
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