P3D Re: Parallax tech

["Gregory J. Wageman" <gjw@xxxxxxxxxx>]

Larry Berlin writes:

>We talk about parallax in stereo images all the time, as well as a
>number of related phenomena, yet I'm unaware if there is an official
>unit of measurement for parallax in stereoscopic images? I know of a
>parallax unit used in Astronomy, but it's pretty specialized to that
>geometry. Is there a different or similar unit for stereo photography?
>If so, what is it's definition?

If you're referring to the "parsec", that's a unit of distance, not
a parallax measurement: a parsec is defined as the distance at which the
PARallax (of a star) would be one SECond of arc, with the diameter of the
earth's orbit as the base.  (It's not a unit of time, either, as misused
by Han Solo in the original "Star Wars" movie.)

Parallax is an angular measurement, and can be expressed in any convenient
angular unit (degrees, radians, etc.).  As such, there's no difference between
astronomical parallax and stereo parallax, other than the sizes of the angles
involved.

You can see from this that the closer the object, the larger the parallax.
Objects at infinity, such as the distant stars, show no parallax with the
largest baseline we readily have (the earth's orbit around the sun).

Parallax angle becomes a distance in stereophotography when the two
perspectives are projected onto the film plane.  We can then measure the
apparent shift of nearby objects with respect to the distant background.