Re: Effective FL after correcting for myopia

[P3D Dr. George A. Themelis <DrT-3d@xxxxxxxxxxxxxxxxxxxxxxxxxxx>]

>>Think of the effective FL as the distance of the lenses to the slide.
>    
>That was my initial approach, but I concluded I was in error based on
>other recent comments that have been made, if I understand them correctly.

In the case of the viewer we have two lenses to worry about, the viewer
and the eyes.  I focus the viewer by bringing the lenses as far away
from the slide as I can and still see a sharp image.  In this situation
my eyes do not need to accommodate, they are relaxed (as if they are
gazing at infinity) and the distance between the slide and the lenses
is equal to the FL of the lenses.  In your case, with your glasses removed
you need to bring the lenses closer to the slide to see a clear (and
sligthly larger) image.  I can do that too.  I can bring the lenses
closer to the slide and force my eyes to accommodate to make the image
sharp.  This is equivalent to viewing something close.  Some people say
that this situation makes viewing macro slides more realistic, but I
find that I prefer the "infinity" focusing where my eyes are relaxed,
even with macro slides.

At the end, the size of the image that you see depends on the distance
of the slide from the lenses.  The closer the lenses to the slide, the
larger the image.  That's the case of short focal length lenses.  The
range of distances that can be taken care by eye accommodation is fairly
small.  The focusing racks in the red button and other viewers are about
10 long and only about 3 mm are used to go from the center to one end.
These 3 mm are a small percentage of the 44 mm FL.

George Themelis


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