"brian" <brianc1959@aol.com> wrote in message news:3c459ba.0308220444.69cb47a@posting.google.com...

> Bruce Murphy <pack-news@rattus.net> wrote in message news:<m2brui5p71.fsf@fuscipes.rattus.net>...

> > Alan Browne <alan.browne@videotron.ca> writes:

> > Naturally, the

> > deviations in rays near the back of a 300mm will be quite different.

> No. A plane parallel piece of glass with a given thickness and

> refractive index will displace the paraxial image plane an equal

> amount for any lens. Focal length has nothing to do with it.

> Brian

> www.caldwellphotographic.com

Yes - but the displacement has a much greater effect on

the new focused distance when the filter is removed with

a 16mm lens than with a 300mm lens...