P3D Re: Cardboarding

[boris@xxxxxxxxxxxx (Boris Starosta)]

I've seen this "cardboarding" effect discussed so often on P3D, now, that
it seems no one really knows what it is, or if they have seen it.  At
least, until now I had no idea that I had ever seen it, or was aware of
what anyone was talking about.

But I finally figured it out.  For the benefit of others, who might be
wondering what this distortion is, I'll now present my theory.  (for the
purpose of this post, any lens focal lengths (f.l.) mentioned apply to
cameras of the 35mm film format).

To me, cardboarding is the opposite of "stretch".  It is seen in slides (or
prints) that are being viewed with a viewer that presents a wider field of
view than the taking lens.  (i.e. the viewer lens f.l. is shorter than the
taking lens f.l.)  The opposite effect, stretch, is seen when the taking
lenses have a wider field of view than the viewer lenses (i.e. the taking
lens f.l. is shorter than the viewer lens f.l.).

Of the several slides I've seen in SSA Folios to date, the most common
distortion is slight stretch, as the most popular camera is the Realist,
which has a shorter f.l. than my 50mm Pinsharp viewer.  This level of
stretch is very subtle and not objectionable.  A few slides have come
through taken with very wide angle lenses (on the order of 24mm f.l.), and
there the stretch is very noticeable.

Only very few slides have come through looking "cardboardy."  Those are a
few portraits here and there, taken with telephoto lenses (70mm f.l. and
up).  The effect, as I have observed it in stereo slides, is always very
subtle, because the f.l. of the taking lens was never that much greater
than 50mm (slides taken with 80mm lenses at most).

However, it is easy enough to demonstrate extreme cardboarding to yourself
using binoculars!  (Once again, through stereo education via P3D, an effect
often observed over the years is clarified to me).  Depending on the power
of your binoculars, your "taking lenses" will have quite a long f.l.,
probably on the order of 200 or 300mm, and a very narrow field of view.
The viewing lenses, or oculars, or eyepieces, will have a much shorter f.l.
and a much wider apparent field of view.  Notice how when you look at
distant objects with binoculars, you can clearly see distance relationships
among them, but they appear relatively flat themselves. This is
cardboarding.

I think the effect generates so much disagreement among stereo enthusiasts
because it is so subtle when it does appear in stereo slides.  Many people
just don't see it, at all.

Respectfully submitted,



Boris Starosta

usa 804 979 3930

boris@xxxxxxxxxxxx
http://www.starosta.com
http://www.starosta.com/3dshowcase

"A lifetime can be well spent correcting and improving
one's own faults without bothering about others."
- Edward Weston

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