Re: Rambling load all right

Kennedy,

A purely educational question. You talk about 'telecentric' lens
designs.
I had never heard of this and looked it up on the web. From what
I could understand, a telecentric lens maps objects of the same
dimension
to the same size on the film plane regardless of object distance from
lens (i.e.
no 'perspective distortion'). Is that what you are referring to or
something different?

Kennedy McEwen wrote:
> In article <341_e.10010$w74.7940@trndny05>, SamSez
> <samtheman@xxxxxxxxxxx> writes
> >
> >So then why do Sigma, Tamron, Nikon, and yes, [lower your head respectfully],
> >even Canon, all use the phrase 'Optimized for digital SLR*' for SOME, but not
> >all of their products???
>
> Because most of the lenses bearing this tag are designed for a smaller
> field coverage, eg. all of Canon's EF-S range, and therefore do not need
> the overhead in glass, mount and movement that is requird to cover the
> full field. Sure, some of them are allegedly closer to telecentric
> design, but then most wide angle lenses for SLRs have to be closer to
> telecentric than those for P&S cameras simply to provide enough room for
> the mirror to flip out of the light path when the exposure is made. All
> such inverse telephoto (where the distance from the optical centre to
> the focal plane is longer than the focal length) designs are partially
> telecentric - it is a consequence of the geometry. For focal lengths
> greater than the mirror height, the non-telecentricity is negligible in
> term of the ray-bundle offset between the filter and the silicon, which
> gives rise to the CA you are sputtering about. In short, the problem is
> negligible in an SLR design in any case, although it may well be a
> problem with P&S cameras, particularly small pixel devices.
> --
> Kennedy
> Yes, Socrates himself is particularly missed;
> A lovely little thinker, but a bugger when he's pissed.
> Python Philosophers (replace 'nospam' with 'kennedym' when replying)

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