P3D Re: Zero magnification?

[Tom Hubin <thubin@xxxxxxxxx>]

Hello George,

> I am trying to
> think of a forula for the magnification factor of the View-Magic
> system.  Can someone help?  Also, what is the viewing distance
> of the View-Magic format?

Unity magnification is arbitrarily defined as something like a viewing
distance of 10 inches. I do not recall the actual distance. But it is a
practical value for as close as you can view with the normally corrected
adult human eye comfortably.

If you move the object to 5 inches from your eye then it will appear to
be twice as large. Hence a magnification of 2. But it will probably be
out of focus for mature eyes (read presbyopic or old). So you need to
wear or view through 5 inch focal length lenses. This will take light
from each point in the object and collimate it as it passes through the
5 inch lens. The human eye then takes the collimated light and focuses
it to a point on the retina. So you now see the object in focus because
each point on the object is mapped to a point on the retina.

The "magnifying" lens actually does not magnify anything. It merely
allows you to focus on an object that is very close to your eyes. 

So a 4x lens has a focal length of 10 inches/4 or 2.5 inches. You hold
the object 2.5 inches from your eye. It appears to be 4 times as large
as when you held it 10 inches from the eye. The 2.5 inch lens permits
you to focus on the object that is much too close for adults with normal
vision to focus upon unaided. 

Now don't quote me on the 10 inches as a basis for this. There is a
standard number but I might be wrong about the 10 inches. Or the
standard might vary depending on the country or the manufacturer. 
 
> PS. I think the term "refractive" is used for a system with
> lenses and "reflective" for a system with mirrors.  Is this correct?

I would say so. There is also a name for a system that has both, like
some telescopes or camera lenses. I think it might be catadioptric but
that is a wild guess.

Regarding zero magnification, that would produce a zero size image. I
did not read the original post but perhaps unity magnification was
intended.

Tom Hubin
thubin@xxxxxxxxx