[MF3D.FORUM:409] Re: fl/30

["Bill Glickman" <bglick@xxxxxxxx>]

David

     Thank you for helping out.... so you are saying, in the example you
cited, 78mm/150mm, or approx. 1/2, * 2.7 or 1.35, right?  So if I use this
figure (1.35)  in place of 2.7 deviation in John B's formula, you feel it
will provide the best 3d base, right?

      What this actually does, vs. using the same 2.7 at all times is, in
the case of the 150mm lens, it cuts the base in half.  It kinda of makes
sense, but I don't have enough experience to know for sure...

Bill


----- Original Message -----
From: David Lee <koganlee@xxxxxxxxxxxxx>
To: Medium Format 3D Photography <MF3D.Forum@xxxxxxxxxxxxx>
Sent: Wednesday, April 05, 2000 9:18 PM
Subject: [MF3D.FORUM:407] Re: fl/30


> 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
> > >
> >
>