Re: Realist --> Holmes

[P3D John Bercovitz <bercov@xxxxxxxxxx>]

Alan Lewis writes:
 
>> Also, I think Holmes stereoscopes have lenses of about 7" focal 
>> length.  (Alan?) 
 
> Yes, 7 7/8" to be precise.
 
>> cameras of the previous century used a little longer lens 
>> because it was very hard to design a sharp normal lens back in 
>> those days: 
 
> Let me approach this from a different angle.  Back in the early 
> days (1800's for us print people; not 1950's as for the slide 
> people) ;-)  they took full sized negatives with view cameras 
> and did contact prints I believe.  If this is true then a 
> "normal" lens would be about 4 1/4" for the Holmes format size.  
> From reading old books from 1900 and from collector books it 
> seems that this was one of the focal lengths used by the stereo 
> cameras.  I think it ranged from 4 to 6" f.l.   Perhaps they did 
> use normal lenses back then after all.  Is my reasoning correct 
> John?
 
Perfectly.  I was unaware of the shorter focal length camera 
lenses.  For those on the list who don't already know it, Alan is 
a major authority in this area.
 
> Now the viewing lenses on a Holmes are 7 7/8", so they are not 
> the same f.l. as the taking lenses.  But in reality the focus 
> distance that most people actually use to focus a Holmes size 
> image is about 6".  So we are getting closer to the taking lens 
> f.l.  Therefore, was the Holmes sized image ever correctly 
> matched to the Holmes viewer lenses?  (Be careful with the 
> answer, we are dealing with the sacred beginnings of stereo 
> photography here!) ;)
 
Heck, I wouldn't tamper with any creation myths.  8-)  Sure, it's 
possible that they were never dead on.  It would be for the same 
sort of reason the Realist red button's lenses are longer than the 
camera's lenses: it's tough to make shorter ocular lenses that 
work for a reasonable price (single element, or cemented 
achromat).  Where the Realist has its problem is that the lenses 
are so short compared to the human eye's size and where the Holmes 
has trouble is that the centers of the lenses are offset from the 
eyes.  In addition (and I don't have the foggiest if it's true), 
I've always suspected that viewer makers erred slightly toward the 
long side because it gives more "pop" to the views.
 
> If I haven't yet left Earth's orbit heading towards the non-
> planet Pluto, then I'll continue...
 
Still lookin' good to me and how about that Pluto?  Sure explains 
a few things, huh?
 
>> To make the 5-perf work in a Holmes viewer, the Holmes viewer 
>> would have to have 1.6*3" = 4.8" lenses and that's tough to do 
>> because the eyes are offset from the centers of the ocular 
>> lenses (unless you can tolerate divergence) 
 
> I'll have to take your word for it on the calculation, I really 
> don't have a firm grasp of the concepts yet.   I think you could 
> use 4.8" "half" lenses if the prism power of the viewing lenses 
> are greater than the standard Holmes 5 diopter (8 diopter 
> perhaps?).  But that might create too much distortion in the 
> image.
 
I was afraid of distortion, field curvature, and lateral color.
Seems like a real toughie to me.  Would love to be shown wrong, 
however.
 
And yes, I get +8.2 diopter.  What power did they use for aphakia?  
I sure can't remember.  I bet it was up to this level but in the 
present case you don't get to look through the center of the lens.
Those lenses were really stretching the art, they were miserable, 
they had to use a special light crown.  Bleah!
 
> Now I'll enter the "theopract" zone where many topics ,such as 
> why bees can fly, reside.  To balance the theoretical/practical 
> aspects of this topic I can only rely on my perceptions when 
> viewing Realist format prints in a Holmes format viewer 
> (everybody with me?).  I have never felt that the Realist format 
> print looks out of whack in a Holmes viewer, but then again I'm 
> not aware of what to look at to determine this.  Any suggestions 
> on what or how to recognize the format difference effect, John?  
> I'd like to understand the effect if possible.  Practically 
> speaking, since I mount my two print views with the correct 
> infinity separation  (relative to the Holmes prism power) my 
> brain perceives the print infinity as real infinity and 
> everything else closer is in the correct spacial relationship.  
> So is it just the magnification that would be affected? 
 
What to look for is stretch (magnification) of the third or depth 
dimension in familiar objects which are close to the camera.  In 
practice, stretch really doesn't matter very much except under 
these circumstances because in the one case, the brain is 
unfamiliar with the true shape of the object and it doesn't matter 
and in the other the brain compensates so very well when provided 
with distance-reduced stereo disparity clues.
 
John B


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