I recall some traffic about digital cameras needing the light to hit the CCD
chip square on, unlike film, this being one reason lenses intended for film
didn't necessarily work well with CCDs. There was some scepticism that this
was a real effect, versus an excuse to get us to buy all new lenses. Anyway,
when I looked at the 4/3 system olympus-pro webpage below, it struck me that
they kept talking up the fact that the lens is "telecentric".
<http://www.olympus-pro.com/eu/en/digitallenstechnology.html> (Use MS Internet
Explorer)
In a message posted a few days ago about the Olympus C50 camera, I cites the
following website on the CCD chip used in the C50, in Panasonic CCD Image
Sensors, 2003: <http://www.semicon.panasonic.co.jp/cat/pdf/A00006HE.pdf>.
It just clicked. The CCD has a little lens over each and every pixel, to
increase the sensitivity despite the loss of active area due to the space taken
by the elecronics surrounding each pixel. These per-pixel lenses will limit
the field of view of the pixels, so now all the talk about telecentric lenses
makes sense.
In this case, what's telecentric is the CCD side of the lens, and what it means
is that the rays (converging cones of light actually) coming from the back of
the lens are always perpendicular to the CCD surface. This requires that the
back aperture of the lens be a good bit larger than the CCD surface. The
reason it has to be larger is that an wide apertures (low f-stops), the cones
of light are very fat, so the lens back must be somewhat larger than the CCD to
get full illumination out to the edges of the CCD.
Joe Gwinn
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