Re: Franks Bean Splitter

[michaelk@xxxxxxxxxxx (Michael Kersenbrock)]

> Just to give you my bias, I took my first stereo pictures with the
> Franka splitter.  I sold it after I bought a Realist stereo camera.
> However, I'm now thinking about getting another, higher quality
> splitter.

I "started" about 15 years ago with a Pentax beamsplitter.  I (now)
also have what I think is a Franka beamsplitter (bought used) as
well as a number of realist format stereo cameras.

I've been planning to write a quasi-rigorous "paper" about tradeoffs
including some technical measurements (including resolution measurements
of my stereo cameras and by beamsplitter(s)), etc.  But I'll put in 
a few off the cuff comments here (Hi George T. :-). That "paper" isn't
a real high priority item.

> That said, I think the splitter is a good way to get started, and even
> go beyond that in stereo photography.  It has two main drawbacks:
> 
>    1) It splits a regular 35mm image area vertically, so you get
>       tall, thin pictures.

Which is appropriate sometimes, but not as often as the square realist format.
It's not as bad as one might think because one becomes creative when
"forced" to live within the constraints.

The thing that causes most need for creativity isn't so much the
aspect ratio, but that with the "effective" image size being that
of a 3.5-perf (or so) image, one's 55mm lens effectively becomes a 
slightly telephoto lens.  *Sometimes* it helps, but mostly it aggrevates
the battle with the aspect ratio.

> 
>    2) It introduces an element of "keystone" distortion, due to the
>       fact that path from front-of-lens to film plane is different
>       in different parts of the image.  My experience, however, was
>       that this was not noticable unless you're taking a picture of
>       a brick wall or some other grid.

It's supposed to be compensated for by the matching viewer 
so any distortion is nulled. Haven't really noticed any though -- but
then I usually use the viewer.

If one goes much out of the recommended f-stop range, one can get
a dark "stripe" down the center of the dual-images.  This really
isn't noticeable in the viewer, seems to be mostly compensated out,
but can raise havoc if one is trying to make dupes and convert to
another format (ahem...).  So one should 'be good' and keep to
the recommended fstop range (5.6 to 8 with the Pentax viewer).

> 
>    3) It may make your camera somewhat bulky.

True for the Franka.  Not for the Pentax unit.

> 
> On the plus side, the splitter has these advantages:
> 
>    1) You can use it on a modern, high-tech camera with whatever
>       up-to-date features you desire.

Including less weight and motor rewind (stereo camera rewinding seem to
get a surprising amount of complaints, although it hasn't been a
problem for me, even for ones without a crank).

There sometimes is a very valuable advantage with the 
modern camera's auto-wind: subjects that want/need a
fast sequence of shots to 'catch' special images in the making.

The time to manually wind to the next exposure with a manual
wind stereo camera can sometimes cause great shots to be lost
due to focusing delay in addition to winding/cocking delay.

> 
>    2) No special processing or mounting is required.  The images
>       are always perfectly registered, etc.
> 
>    3) The images are always perfectly synchronized, and get the
>       identical exposure settings.
> 
>    4) You can take it off and shoot "mono" pictures on the
>       same roll of film.

The single biggest single advantage to the beamsplitter is available
shutter speed.  The most obvious advantage is for subjects that are
moving, but there also is an advantage for image sharpness when one doesn't
have a tripod or even the time for "proper" hand-held stabilization techniques.
My beamsplit images are very much more consistently *sharp* in opportunistic
shots.

> The Franka splitter I had was a lightweight plastic thing with rear
> surface mirrors, which means you get additional distortion, blurring,
> etc. because the light has to travel through the glass of the mirrors
> twice.  For about the same price, you can get a somewhat fancier
> imported splitter with better mirrors, etc. from Joel Alpers and, I
> believe, some others on this list.  Their splitter can also be used as
> a viewer for split slides.

The construction of the Pentax beamsplitter is an order of magnitude better
than my other beamsplitter.  The Pentax unit is much more compact, easy 
to keep "even", front-surface mirrored, and has a snap cover that covers the 
open end. Even though mine is 15 years old, it's still clean and scratch free.

I talked to a Pentax rep about it at Camera World of Oregon at one of their
recent sales, and she said that it's still in their catalog but it's
a OEM'ed product now -- which accounts for it's obscene price (she didn't
use the word "obscene", but the price she spoke was just that).

They used to be under $100 including the viewer.  But then Realist red
button viewers used to be under $20 too I suppose.....

Mike K.


P.S. - All in all I prefer the Realist format for general use, but still
       will use my Pentax beamsplitter for special applicatons.  I'd really
       like a high resolution digital stereo camera with real-time AI-based window
       correction (etc).  In time it will come.


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