P3D Franka, Stitz, closet

[SHIELDS@xxxxxxxxxxxxxxxxxx]

Hi!

There is absolutely nothing like actually having something in your
hands to examine it.

My long-awaited Franka beamsplitter arrived today, and I took it home
and gave it a good checking out.

It's going to be very simple and straightforward to install front-surface
mirrors. The old ones may have to come out in pieces--I'm going to wear
eye protection--as they seem very tightly glued in place.

It is a 55mm unit, with step-down rings supplied for 52mm and 49mm.
There is also a 55mm rotational adapter supplied, apparently a
design intended for a polarizing filter, so you can keep the splitter
level. If you ever feel the rotating adapter puts the splitter too
far out in front of the lens, the 55mm-to-bayonet adapter that actually
attaches to the splitter itself can be carefully rotated--it's very
stiff as it's not meant to rotate as such. All of this stuff IS made of 
plastic--it's very nicely made, but I'm sure you will come to grief if 
you force the threads in the wrong way or exert too much pressure on it.

What struck me at once was the sheer size of the unit. I had thought
maybe product illustrations (showing it with a Nikon, I believe)
had purposely exaggerated it, but towards the end, even before I
got it, I was thinking it really was big. And it is.

The mirrors are half again as large as the Pentax or the Stereotach.
This could mean it will handle slightly wide-angle lenses. I haven't
tried it as I really want to go toward telephoto. What significance
this has for larger formats I'm not sure. For everyday use I much
appreciate the smaller size of the Pentax/Stereotach.

MORE IMPORTANT, the stereobase is a whopping 105mm, as compared to
70mm in the Pentax and Stereotach. I can actually get reasonably
on-film with an 85mm lens, without any mirror adjustment. The
images are too far apart, but you could take a picture and make
adjustments in mounting, if you wanted to. With a little toe out,
I am confident this will work. The unit as it comes adjusts only
for toe-in, but since it is a very simple, straightforward plastic
box, it will be easy to cut part of it away to make the mirrors
toe out. Judging by recent discussion on the list, I would say
85mm is as far as I can go without too much toe out, although
I have a 100mm lens and will try that when I get the modifications
made and get another adapter (the 100mm lens has 58mm filter
threads).

I would say the large stereobase makes it more suitable for
a larger format, if anybody else wants to try it, but
there's a lot of math involved here that I don't really
understand.

The mirrors handily adjust to do closeups, but if I use a +2 or
+3 lens to get really close (on the 85mm), the keystone distortion is 
enormous-- I'm toeing in from a 105mm stereobase! I can probably
do O.K. if I get the adapter and use the 100mm lens, but I think
the 85mm will do just as well (except for adjustability--see below)
using the Stereotach's shorter stereobase.

I think the Stereotach on the 85mm lens is going to make more
distortion-free closeup shots, as I don't have to toe in at all.
(There is some of the beamsplitter's own inherent keystone
distortion.)

Sadly, I do have to conclude that the Stitz, with its smaller
stereobase AND adjustable mirrors, would do a better job on
flowers IF you are using a normal lens or if you want to use a wide
range of magnifications.

But, in a reasonable range, my Stereotach on the 85mm will do just as well!

So the Franka is going to be my telephoto splitter. Since it has
a 105mm base, it will be much BETTER than the Stitz for this
purpose.

AND MY CLOSET IS NOT EMPTY!  :-)  I have an old Stereotach in
there, too, in pieces. I may need to get some new mirrors cut
for it, but then I can fabricate some adjustable "wings" like
the Franka uses (a very simple, straightforward design), and
toe in with a 70mm base just like the Stitz!

Thanks to everyone who responded with ideas about beamsplitters,
and special thanks to Milan Zahorcak, Gary Schacker, George Themelis,
and Steve Berezin for their help in acquiring my four beamsplitters!

Mark Shields

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shields@xxxxxxxxxxxxxxxxxx
http://www.netcom.com/~northws1/stmatt.htm
"Let the little children come to Me," Jesus said, "and don't keep
 them away. The kingdom of heaven belongs to such as these."
  -Matthew 19:14



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