Re: Effective FL after correcting for myopia

[P3D <PTWW@xxxxxxx>]

George Themelis advises:

>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.
Greg Kane mentioned, and John B. confirmed, that FL of a stereoscope's
lenses can be measured by using the lenses to focus the sun on a piece
of paper.  See digests 1812 and 1817.  (I presume this means you then
measure the distance from the lens to the piece of paper to obtain the
FL.)  I did this with the lenses of the 3Discover and obtained a
measurement consistent with the numbers mentioned on the list as
approximating the FL of those lenses.  However, the paper must be placed
significantly farther from the lenses than the image-to-lens distance of
a 3Discover in use.  This is why I concluded that image-to-lens distance
is not the same as the FL.  My guess is the discrepancy arises due to the
sun being at infinity, while the 3Discover image is very close to the
lenses.  (I imagine watching me struggle with these concepts must be
rather amusing--or tiresome--to the tech-3D whiz bangs!  Sorry 'bout
that!  I just use these things, I never understood why they work!)
Am I just incapable of making good measurements, or can someone help
explain this?

>If you inrease this distance when your refocus the viewer then you
>inrease the FL and visa versa.

I agree regarding the directional effects of the shifting.

>>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! ;)
>
>I believe I have the 1st prototype ;)

Ah, but mine is no doubt *far* cruder than yours and therefore likely to
have much greater collectible value. ;)

>Still, you have not mentioned if you like (prefer) the strech or not!

No, I do not prefer the stretch.  For the first couple slides, the
unfamiliarity of the reduced stretch made the image seem less enjoyable.
This is probably similar to the experience George described in digest
1746, where he stated he initially could not stand his first red button
viewer, because he could not get used to the shorter FL.  But I very
quickly came to prefer the visual effect of being closer to the window
and the sensation of being closer to being in the scene.  One downside
is the grain in the sky, apparent even with ISO 50 (32?) Fuji chromes.

I have never cared for the stretch from viewing projected slides from
any distance, either.  I even suspect that if I cannot adjust focus on
a red button viewer enough to use it without my glasses, I will prefer
the reduced stretch of my PT'd 3D viewer.  I better hold off on that
laser cornea shaping or I'll be stuck with all the stretch of 44mm FL
Red Button viewer lenses! ;)  Too bad my myopia has already improved
so much on its own with my advancing years.  ;)

I'm ready to try even closer to ortho!  George, do you think you can
get your hands on some new old-stock Gold Button viewer lenses?  Or
does anybody want to swap their Gold Button lenses for Red Button
lenses? ;) ;)

Paul Talbot, getting unstretched


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