Effective FL after correcting for myopia

[P3D <PTWW@xxxxxxx>]

If a near-sighted person removes his glasses and refocuses the lenses
of a slide viewer to bring the slide back into focus, what impact does
this have on the effective focal length of the lens (if any)?  My guess
is the effective FL is lowered (the lenses have to move closer to the
slide to focus the image on the myopic person's retinas.)  How would one
go about calculating the new effective FL?

I did an experimental conversion of a $3 viewer to use a pair of DrT's
"new old stock" Revere viewer lenses.  The result might be called a
PT'd 3-$ (or using D for DDDolar, a PT'd 3D) ;).  Advance notice:
this 1st prototype, which undoubtedbly will be extrememely valuable
to collectors in the future, just might show up on Sell-3D someday! ;)

Unfortunately, significant chunks of the corners of each image are
lopped off when I view a 5P Realist image with these lenses (which is
*not* a problem with an unmodified $3 viewer.  Do eyeglass wearers have
this problem with Red Button and other Realist format viewers with a
shorter FL than the $3 viewer?)  With my PT'd 3D, there is a fairly wide
focus adjustment range; I was able to adjust the lenses in far enough
that I could view the image without my glasses.  I can see a very
noticeable reduction in the stretch of my slides using the PT'd 3D in
this manner, as compared to an unmodified viewer.  I also think I can
see a difference in the stretch of the scene from the PT'd 3D with my
glasses on as compared to the same scene with the lenses pushed in
closer and my glasses removed.  I'm trying to get an objective measure
of the realative magnitude of these two effects.

Paul Talbot


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