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[OM] Digital camera uses telecentric lenses

Subject: [OM] Digital camera uses telecentric lenses
From: Joe Gwinn <joegwinn@xxxxxxxxx>
Date: Mon, 23 Jun 2003 22:18:31 -0400
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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