Re: [tech-3d] Re: Stereo Base Calculation With a $20.00 Handheld

["David Lee" <koganlee@xxxxxxxxxxxxx>]

Boris wrote:

> I have a question.  Why does a viewing device focal length (i.e. a viewing
> geometry) even come into play?  On film deviation is the product simply of
> camera optics, stereobase, and near and far point.  Why the viewer F.L.?
> Granted, the viewer optics affect the perceived depth, but it certainly
> does not change OFD.

The viewer focal length does not change the OFD, but it does change the size
of  a given deviation on the retina. The shorter the focal length of the
viewing lens the greater the deviaton (on the retina). And in fact it is
proportional, which is why it can be used as a factor: (FL viewing lens)/(FL
taking lens).


>
> (And for my application, in shooting for anaglyphs, there is no viewer
F.L.

There is always a viewer focal length, it is simply the distance from your
eyes to the image. In regular photography there is a term, "proper viewing
distance," which tells you how far to get from a print so that the objects
in the image will appear proportional in size (similar to "ortho" in
stereo). The way to figure the proper viewing distance for a print is to
multiply the enlargement factor times the focal length of the taking lens
(although in practice it actually makes more sense to multiply by the focal
length of the normal lens for that format). For example, let's say you have
an 8 x 12 print from a 35mm negative (an 8 times enlargement) times 2 inches
(the normal focal length) equals a viewing distance of 16 inches. Does this
mean that you have to view the print from 16 inches? Of course not, but it
does give you a starting point from which to base other decisions. If you
don't like the deviations that you're getting using this data, then modify
it until you get something that works better for you.

My preferred method of presenting stereo images is with a large mirrored
viewer (of my design and manufacture) which allows one to vary the viewing
distance (focal length). Nevertheless, I still base my deviations on the
normal focal length and have found that it works just fine.

> There may be a print size, and a viewer's naked eye distance to print -
> that would form the equivalent to a viewer's focal length.  But to shoot
> for a specific size anaglyph print, and therein a specific deviation on
> paper, I want to predict and set up a shot simply for a particular
> deviation on film.  Viewer optics are not otherwise considered.)

As mentioned above, when you think you are not using the viewer focal
length, you are de facto using the normal focal length in your calculation.
The factor is 1/1, that is, viewing lens FL and taking lens FL are assumed
to be the same unless you put in a different factor.

David Lee


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