Re: [photo-3d] Digest Number 249. Base calculator

[Olivier Cahen <o_cahen@xxxxxxxxxxxxxxxx>]

	This limit of 1.2 mm is not at all an absolute limit, even with the 35
mm film. It was set for cameras with very short focus such as the
Realist, and it is linked to the fact that the viewer is supposed to
have the same focal length as the camera, since with the same focal
length you see the image exactly as it was shot with the camera.
	I personally use a RBT 7-perf camera with 50 mm fixed focus. I project
my images (at home) on a 5' wide screen, with an 40x image magnification
from the film (24x30mm becomes 96x120 cm). The seat for viewers is
located two meters away from the screen. These are the perfect
conditions for best stereo viewing. In these conditions everyone can
perfectly tolerate deviations up to 1.6 mm. The stereo window is located
on the screen and the background is deviated by 40 x 1.6 mm, i.e. 64 mm
on the screen, so that it is viewed with parallel eye axis, that is
quite normal.
	The maximum deviation should be matched to the viewing conditions. A
current simple formula is that the maximum deviation of the magnified
image should not exceed the thirtieth of the viewing distancce. This
formula leads to 1.6 mm on the film for my viewing conditions, or 1.2 mm
for a 35 mm focus viewer for your slides made with a Realist, or 1 cm on
the screen for the digital images that you are watching on your computer
from a 12" distance.






 
> Now the real reason for posting this message. Being new at Stereo can anyone tell me if p=1.2 is actually a constant to be used for all lenses, or does it change somewhat with focal length?  How was p arrived at? Is it a true calculated value or an empirical. Was it derived from astronomy? If it changes with FL how so? I cannot see how it could increase linearly at the slope of my focal lengths otherwise b would always be the same. Or does p actualy decrease as FL increases? How about it you experts?
> 
> Joseph F. Valvo
> 
> experimenting wit