[rollei_list] Re: Really Off-Topic, paraxial lenses for digital cameras - ultra wideangle lenses
- From: Emmanuel Bigler <Emmanuel.Bigler@xxxxxxxx>
- To: rollei_list@xxxxxxxxxxxxx
- Date: Thu, 02 Jul 2009 15:12:52 +0200
Just to add a few ideas to Richards' introduction to telecentric lenses.
First I doubt that any photographic lens specifically designed for
digital photography is actually telecentric.
A telecentric lens needs a lens diameter equal to the diagonal of the
format to be covered. In this sense, the four-thirds standard was
probably designed with an ovoersized bayonet diameter.
Hence telecentric medium format lenses would be really huge.
Apart from profile projectors, telecentric lenses are supplied as an
option for some optical scanning instruments ; either image scanners or
machinery that focuses and scans a laser beam to do machining or
engraving for example. For example Rodenstock's scanning lenses can be
supplied as telecentric or not.
Retrofocus lenses that everybody uses in reflex cameras are somewhat
intermediate between a quasi-symetical lens like a tessar or best : an
apo-ronar (whicc is perfectly symmetrical). Those lesnes behave almost
like a single lens element with the f-stop at the center of the lens
element.
The major difference between a quasi-symmetrical lens and a telecentric
lens is in the rendition of out of focus images.
To put it simply in a few words, the usual model of central perpective
describing the behavior of a pinhole camera is also perfectly valid for
your planar, xenotar, tessar or xenar in your R-TLR.
But for your distagon, forget it, the projection properties of out of
focus images cannot be exactly simulated with a pinhole camera !
With a really telecentric lenses, it is even more strange.
--
Emmanuel
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