Reply to Question

[P3D emdesher <emdesher@xxxxxxxxxxxxxxxxxxxx>]

Why are there a range of mask apertures, or why doesn't a size 9 shoe fit 
everyone?

There are several interrelated aspects to this topic, and for this
discussion, lets start with a standard stereo camera that produces a film
pair exactly 5 perforations wide.  A key mask design decision was whether
to adopt a consistent center to center distance between different
apertures, or to vary this spacing and thereby be able to maintain the
aperture width.  The Albion mount for the "Realist" format, for example,
only comes in one size.  In America, the choice was to maintain a standard
62.3mm spacing, regardless of aperature choice, in order to eliminate the
necessity of readjusting the horizontal projector controls for each slide. 
If you would place an EMDE normal, medium, and close-up mask over each
other, you would see that the mask center is indeed constant, while the
sizes of aperatures are different.  Many stereographers observing this,
have assumed that this is what makes a close-up different from a normal
mask.  In this example, the close-up has a horizontal dimension of 19.6mm,
compared to the 21.6mm of the normal mask, or 2mm less.  However, it's the
relationship of the spacing between the centers of the film pair TO the
fixed mask centers that determines the location of the perceived stereo
window.  It's not the distance between mask apertures or their width!  Why
then have different size masks?  In order to have the near point, or
closest definable object, at or close behind the stereo window (ignoring
some exceptions for now) for comfortable viewing, it is often necessary to
realign the film pair.  This could easily result in light gaps on the
sides, and in the case of the "Realist" format there is precious little
side edge margins to work with.  The mask acts as a trimmer by blocking
off non-stereo side edge images and covering up areas where light gaps
would be if a normal mask was used.  The close-up mask has changed the
imaginary location of the stereo window closer to the observer, without
having to readjust the viewer or projector.  The trade off is the narrower
aperature size.  Next, 7-perf, if interested.  Dennis Sherwood


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