David Richardson wrote:

> In the discussion of the wide modified tri-lensed
> cameras, one seems to remember the mention of lenses
> being individually focused. Is this truly so?

Yes. If you will look in the archives for message 1641, you
will find a discussion where Peter Thomson discusses focusing.
http://tech.groups.yahoo.com/group/MF3D-group/message/1641

In short, each lens focuses by hand and are not coupled. The
way I picture this working is either by zone-focusing in
pre-marked areas, or by focusing with the finder-lens and
then transferring that setting to the taking lenses. In
either case, the builder of the camera will need to
calibrate the focus of the camera lenses and inscribe
markings on the lens board and lens barrels so the focus can
be set later.

> If not being focused via the viewing lens, one presumes
> having to place a ground glass plate at the prime focus
> point and focus each lens with the help of a lupe of some
> kind?

This would be part of the building process. The finder-lens
will be focused on its glass and the taking lenses will be
focused on a piece of ground-glass at the film-plane.
Matching marks can then be made on the lens board/barrel
interface to find this spot again.

To later focus at a different distance, the finder can be
focused and its position relative to the index mark can be
noted. The taking lenses can then be similarly positioned.

> Just trying to wrap ones mind around how a user would get
> focusing done if the top lens isn't the guiding optic of
> such a optics suite. Going back to a dark focus cloth (as
> we all did for large format work) seems so ponderously
> slow as to be not likely these days.

When I started working on my TL120-55 project, Sam had built
his fixed-focus camera and I wanted to up the ante and make
my focusing. I was working on a design incorporating the
focusing helix from Canon QL17 cameras. As I started doing
the math to establish how much travel I would need and what
focus-range I could get, I figured out that the lenses would
hit the shutters if focused at distances greater than 3.5m.
This meant that unless I modified the lenses and the camera,
all my work would create a camera with a maximum focus range
from 0-3.5m. Since I didn't want to mill my camera, it just
wasn't worth the effort to me.

'tis easier to just fix the focus at 3.4m. If you shoot at
f/22, everything from 2m-infinity will be fine. If you shoot
at f/16, I think it's good to 15m rather than infinity. Load
the camera with Provia 400X and fire away.

Can you envision a scenario where focusing a TL120-55 to
something other than hyper-focal would be advantageous?

> Clearly I'm missing something in this limited production
> stereo array. Someone please help this clueless 3D fan
> understand this variation of TL-120.

Please note that what we're describing was never a
production camera. It is in no way designed, blessed, built
or supported by 3D-World.

I produced seven lens boards to convert TL120-1 cameras into
what I called the TL120-55. I own one and of the other six
boards, I think five cameras were built. With the exception
of one fully populated and tested lens-assembly I sold, all
others were built by their owner on the board I sold.

Sam Smith produced his Mamiya-equipped TL120. The lens board
and lenses he created and assembled is currently for sale.
If you want to see how it all works, contact Sam to buy it.
It is _by far_ the easiest way anyone will now put 55mm
lenses on their TL120.

Peter Thomson produced his focusing, Mamiya-equipped TL120.
I have heard from Peter for a couple of years, but I suspect
he is still lurking here. Speak up, Peter, and tell us more
about it!

--
John Thurston
Juneau Alaska
http://stereo.thurstons.us