P3D Re: Stretcho-TDC

["Dr. George A. Themelis" <DrT-3d@xxxxxxxxxxxxxxxxxxxxxxxxxxx>]

Hi Michael!  

I read your entire posting. (Honestly! :-))  You must be extremely
sensitive to stretch.  What kind of pictures were you looking at?
All of them people at close range?  I am rather surprised by your
reaction.  I remember I noticed stretch in a picture of great
grandma holding our newborn daughter.  I thought it was terrible
but a few years down the line I cannot see the stretch any more.  
Plus, other pictures (not people) always looked fine.

Minor correction.  You wrote:

>the image which is already small by virtue of having 5-inch lenses 
>is even smaller given the distance away that I am...

Note that the image is small not because of the 5-inch lenses
but because of the distance between screen and projector.  You can
make it as large as you like by increasing this distance.  But it is
true that you are stuck away from it as the lonely projectionist.

I personally like the 5" lenses.  One misconception is that the 4"
lenses are brighter.  They are not.  Brightness is a function of
image size.  The larger the image the dimmer it is. 

>1. What, if any, is the relationship between the focal length of the
>projector lens and the amount of z-axis distortion?

No relationship!!!  This z-axis distortion does not depend on focal 
length.  It only depends on the ratio of image size to viewing distance.

>2. How is it determined what's the best "ortho" seat in the house? 

Usually this is at the center and equal to the diagonal of the 
projected image (assuming that "normal lenses" are used and the 
Realist lenses are normal for the image size.)  Note:  "best ortho" 
sounds funny.  Ortho, period.  Whether it is best, it is a matter of
interpretation, personal taste, etc.

George Themelis

PS. You can work out the geometry of stereo projection using similar
triangles.  The ration of the FL of the lenses to, say, image-on-film
size, is equal to the ration of distance of the projector from the
screen to the image-on-screen size.


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