Re: ortho seat question (was: Stretch noticed by novice)

[P3D Michael Kersenbrock <michaelk@xxxxxxxxxxxxxxxxxx>]

> > Are you sure he was close to the "ortho seat"?  Whenever I've done
> > the calculation for my 5" lens'd setup one almost has one's nose
> > to the screen to be ortho.  Well, not quite, but close.  A few rows
> > in front of the projector sounds like a stretched spot.
> 
> 	Lets say you had two projectors, one with 5" lenses and
>     one with 4" lenses.  If you set up both projectors so the
>     on screen images are exactly the same size and in the same
>     position, would the ortho seat be different for each projector?

Yes it's true that the practical dependancies are the camera's focal length
and the size of the projected image.

I recall my rule of thumb (taken from the calculations done long
ago) for use when I attend a screening, the ortho position is
roughly 1.5 feet away from the screen for each foot of screen
image height.  Easy to guestimate where it is w/o forcing my
brain to do much work.  :-)

It makes assumptions about nominal camera f.l. (probably around 35mm),
and format.  Recall it's a "rule-of-thumb" not a detailed analysis
of the camera lenss, film size, and everything else.

For my screen at home, the ortho spot is about 5 feet from
the screen, and this is roughly verified by the walk-around exercise 
I mentioned yesterday.  Larger screens will have the point further
back, but not anywhere near the 5" f.l. projector.

That's one reason I'd like to get 4" (or shorter) lenses for
my Triad if I can ever find any available (and for less than
a fortune).


>     If not, then the ortho seat must depend only on image size, and
>     not projector distance.

True, but for a given projector's focal length (and the same slides),
there's only one projector distance for any given screen image-size
so the two are "interchangeable" in a practical sense.

With 5" lenses, for an ortho seat where the viewer's nose doesn't
touch the screen, the projector is back quite a ways.  :-)

Mike K.

> 
> 


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