Re: [photo-3d] Beam splitters vs. Image splitters

["John A. Rupkalvis" <stereoscope@xxxxxxxxxxxxx>]

----- Original Message -----
From: "Brian Reynolds" <reynolds@xxxxxxxxx>
To: <photo-3d@xxxxxxxxxxx>
Sent: Tuesday, January 09, 2001 10:52 AM
Subject: Re: [photo-3d] Beam splitters vs. Image splitters


> Bryan Mumford wrote:
> > If you are willing to educate me further, I'm curious why beam
> > splitters are used in photography. Why do you wish to capture the
> > same scene on two cameras?
> >
>
> By using a beamsplitter you can get the optical axis of the two
> cameras arbitrarily close.  This means that you can use a very small
> inter-axial distance to take macro stereo photos.
>
> For non-stereo uses a beamsplitter allows the camera and another
> instrument to see the same scene.  Some Canon cameras use a
> beamsplitter (or the moral equivalent) instead of a reflex mirror.
> One beam of light goes stright through the camera to the film and the
> other goes up to the light meter and viewing prism.  This scheme means
> that they don't have to worry about camera shake due to a moving
> mirror, or the delay in firing the shutter after the mirror moves out
> of the way.
>
> --
> Brian Reynolds                  | "Dee Dee!  Don't touch that button!"
> reynolds@xxxxxxxxx              | "Oooh!"
> http://www.panix.com/~reynolds  |    -- Dexter and Dee Dee
> NAR# 54438                      |       "Dexter's Laboratory"
>
>
Good description.   The Canon type of beamsplitter is called a pellicle.  It
is very thin so as to avoid a "ghost" image from the back reflection off of
the second surface.  I believe that these were AR (anti-reflection) coated
as well.

Pellicle type beamsplitters are rarely used in stereo photography because
the large sizes required would make them very expensive and extremely
fragile (a bad combination, I am sure you will agree).

They rely on AR coatings for the same reason, as the thicker glass does not
pose as much of a back reflection problem at the lens distances typically
used, and a magnesium-fluoride single coating is usually sufficient at 45
degrees incident.

JR