Re: Re. Re. Viewer Questions

[P3D Dr. George A. Themelis <fj834@xxxxxxxxxxxxxxxxxxxxx>]

>Let me apologize to Dr. T for seemingly casting aspersions
>on his beloved viewers.

No apologies needed... I use viewers to view images... I am not tied
emotionally to them... Since there is a lot of physiology (not only math)
involved in the way we appreciate the things we see, I recommend using the
viewer that works best for you, no matter what the theory tells us...

>However, I'm still a bit confused on FL. With a camera
>lens the longer the FL the smaller the angle of view 
>thus apparently enlarging the image. Dr. T's reply
>suggests the longer the FL of the viewer the smaller
>the image. What have I missed here, which had me
>concluding the smaller image of the $3 viewer was 
>due to a shorter FL.

I see what the confusion is... In a camera the lens projects the real scene
to a piece of film.  A shorter FL lens will show more (has a wider field of
view) thus apparently reducing the image.  In a projector however the lens
projects the image from the film to the screen.  A shorter FL lens will
project the same image to a larger area, thus apparently enlarging it when
intercepted by the fixed-size screen.  In a viewer the lens projects the
image to the eye.

A better way to understand this is to consider the distance of the eye from
the image.  When freeviewing, the distance that the image is held from the
eyes is effectively the FL and no lenses are used since the eye can do the
focusing.  When you bring the image sufficiently close to the eye, there is
a point where the eye cannot focus it any more and you need lenses to see
it sharp.  The closer you bring the image to the eye, the larger it will
appear... the lenses only doing the focusing... Thus, a short FL viewing
lens lets you see a larger image just because your eyes are positioned
closer to the image.

Hope this helps... -- George Themelis


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