P3D Re: The Continuing Saga of "VM Virtual Viewer with achromats"

[Tom Hubin <thubin@xxxxxxxxx>]

Hello Al,

> I guess you could move the lenses inward/outward about a half-inch, which
> may be just enough for people with -2.00 to -6.00 eyeglass prescriptions.
> It's been a long time since I was an optician, so my estimates may be off
> somewhat.

A -2.00 diopter lens has a focal length of 1000mm/(-2.00)2=-500mm. That
means you can see clearly something that is 500mm (19.7 inches) away
from your eye. If you use a 50mm lens then placing the slide 45.45mm
from the 50mm lens will produce an image 500mm away from the lens and on
the same side of the lens as the slide. That is 4.55mm closer to the
slide than standard.

A -6.00 diopter lens has a focal length of 1000mm/(-6.00)=-166.67mm.
That means you can see clearly something that is 166.67mm (6.56 inches)
away from your eye. If you use a 50mm lens then placing the slide
38.46mm from the 50mm lens will produce an image 166.67mm away from your
eye and on the same side of the lens as the slide. That is 11.54mm
closer to the slide than standard.

Instead of using a 50mm lens with a power of 1000D/50=+20 diopters. You
could use a +14.00 diopter lens which would be the same as the +20D lens
combined with a -6.00D correction. This would be a 1000mm/14=71mm focal
length. So a viewer with a 70mm lens located about 50mm from the slide
should work well for somebody with a -6.00D prescription.

Tom Hubin
thubin@xxxxxxxxx