Re: Double depth

[T3D Steve Spicer <s.spicer@xxxxxxxxx>]

Dr T wrote:
>Sorry John... I don't understand this.  What do you mean by
>"double depth"?  Have never heard this term before in my 3d
>life.  Am I missing something?

Then Bob Aldridge wrote:
>Surely it came from PROJECTION.
>If you decide to put the window at the same distance as the screen (i.e.
>the frames are co-incident) then extra depth in the scene will give rise
>to excessive separation of the infinity points...

Ferwerda describes the double depth technique in his book. I think I can
remember the details, and will try to summarise:

                              Aperture    Infinity      Maximum Film
                              Separation  Separation    Deviation
                            ------------------------------------------
Conventional Stereo Mount:    62.2mm      63.4mm        1.2mm

"Double Depth" Stereo Mount:  61.0        63.4mm        2.4mm
                            ------------------------------------------

As Bob says, _if_ the aperture frames are coincident on screen, then the
infinity point will be excessively separated. However, I would expect that
to successfully project a "double depth" slide, that one should not adjust
the aperture frames to be co-incident. Instead, the window should be left
so it appears in front of the screen surface.

I think the crux of John's question is: does having the window in front of
the screen surface cause any discomfort for viewers?

I've seen double depth slides in hand viewers, and like John I think they
look fine - no problem to view. I have made my own double depth slide
mounts by masking the outer edges of each aperture in a conventional mount,
for shots I have taken with too much depth range in them (eg: stuff at 4
feet and infinity).

My own feeling on projection is that double depth slides are viewable in
projection, but there is a little extra effort required to look at them.
However, my eyes are used to such things, and for the less experienced
viewer, this may translate into actual discomfort.

Side Note: I think the Stereo Realist Manual actually discusses stereo
mounts that have differing aperture center distances (were they called
"Permamouts"??). From the way it is written, I seem to recall deducing that
Realist started out at first with mounts that had varying center distances
(less aperture separation and smaller apertures for close up mounts), but
soon progressed to a constant center distance like all modern mounts are
(with smaller aperture sizes and near-point separation not less than 62.2mm
for close up mounts), and that these were superior in projection.

Steve Spicer


------------------------------