[MF3D.FORUM:407] Re: fl/30

["David Lee" <koganlee@xxxxxxxxxxxxx>]

Simply multiply the result by an additional factor ((focal length of viewer
lens)/focal length of taking lens), in this case (78mm/150mm) which is
approximately 1/2, and this will result in the same on film deviation. It's
really pretty intuitive when you think about it. A lens that is twice as
long is magnifying the image (and the resulting deviation) by a factor of 2,
therefore you need to diminish the base by a factor of 2 if you want the
deviation to remain constant. And it works in practice. I have done it many
times. The general rule is: longer than normal lenses need a reduced base to
give a constant deviation while wider than normal lenses need an increased
base to give a constant deviation.

This, by the way, seems to be the opposite of the PePax principle which
McKay used to try to cancel 2 deviations with each other. He increased the
stereo base in exactly the same proportion as the focal length of the taking
lens was increased (p. 61 of Three Dimensional Photography by Herbert
McKay). However, he was not concerned about on film deviation, only with
making the distortions cancel each other (which, of course, is impossible).
The PePax principle is not necessarily incompatible with the factor I
mentioned in the first paragraph though, because in many cases a longer lens
will cut out much of the foreground, thereby making the near point farther
away, necessitating the use of an increased base even though the factor has
been halved.

David Lee



> John B
>
> Oh no.... there goes using other fl lenses with this formula...
>
> Do you feel that 2.7 can ruin shots stereo shots with much longer fl
lenses
> than the viewer has?  I don't know if I have your mathematical wizardry to
> tackle this one... any body else think they do?
>
> Regards
> Bill
>
>
>
>
> > >      What happens when the camera lens fl does not match the viewer
fl.
> >
> > First answer is that I hope you don't have anything recognizable and up
> > close in the scene because the depth dimension is going to be wrong
> > depending on the magnitude of the mismatch.
> >
> > >For example, camera lens is 150mm and the viewer lens is a constant
78mm?
> > >Now what do I use for the deviation figure in your formula?
> >
> > I don't know - it's a good area for research.  My formula only covers
the
> > cases where the lenses match.
> >
> > John
> >
>