Re: Realist --> Holmes

[P3D <LDAEnt@xxxxxxx>]

>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?

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!) ;-)

If I haven't yet left Earth's orbit heading towards the non-planet Pluto ,
then I'll continue...

> 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.

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? 

Alan Lewis 



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