Re: Holmes focal length

[P3D <LDAEnt@xxxxxxx>]

>I have three Holmes viewers:
 - A plastic-lens reproduction
 - A Red Wing viewer
 - A restored real antique stereoscope

>The focal lengths are all different. If I recall, the plastic lens one has
>about the focal length you mention, but the other two are < 7". The antique
>may be about 6 3/4". I posted these 2-3 years ago.

It sounds, from your values, that you may be measuring the distance from the
card to the lens after you focus it with your eyes (through the lens).  You
will get these distances with this method.  But the focal length of lenses is
measured by projecting a point at infinity through the lenses onto a card.
 When checked this way the sliding stage will be way at the end of the
tongue, almost falling off. 

Results that I get (I use a scale and my point isn't quite at infinity, so
accuracy is probably about 1/16):

Antique Keystone: approx. 7 7/8"
Antique U & U : approx. 7 3/4"
Antique no-name: approx. 7 7/8"
SaturnScope I (my current repro. scope): 7 7/8"
Red Wing (from an actual lens element): approx.  7 7/8"
Stereo Biopter (sold new from Reel-3D): approx. 8"
Taylor Merchant plastic molded lens: approx. 7 7/8 to 7 15/16"
Plastic lorgnette (either RCI, AOTech, or Added Dimen.): approx. 10"

The current (modern) standard for Holmes lenses is 5 diopter, and this is
based on the most common antique lenses.  There most certainly will be a few
scopes that don't fit within this standard.

Note that when you focus your eyes through any scope that the distance from
the card to the lens will be less (by 1 to 2") than the actual focal length
of the lens, and that this focus distance will change slightly relative to
the centerline distance of the views. 

Alan Lewis




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