Re: Large Format Camera shift

[P3D John Bercovitz <bercov@xxxxxxxxxxx>]

Michael Nankin asks:

> I'm thinking about (just thinking) trying some stereo with my 8x10 view
> camera. How do you view two 8x10's? I don't know, I'm just thinking. 

With a Wheatstone viewer?  All you have to do is reverse the views.

> But here's the question: Let's say I'm doing table top work. I take one 
> view and now, as I prepare to shift the camera for the second view, do 
> I shift only the lensboard? Only the camera back? Or the whole camera?

Tabletop with an 8x10; what a concept!  OK, here's what you do.  You want
to shift the lens to get the two viewpoints.  You want to shift the back 
even farther to set the window.  Let's say your tabletop doesn't have too 
much depth so you can use the full 65 mm of stereobase (it's surprising how 
often you can actually do this so don't laugh).  So that establishes the
lens shift.  

Now we move on to shifting the back.  In general the goal is to set the 
stereo window a little closer than your closest object.  So now we need to 
know a little more about your setup.  Let's say you're using a 200 mm lens, 
the closest object is 800 mm away, and the farthest object is 1200 mm away 
giving you a best focus at 960 mm.  (You're going to have to stop down to 
about f/128 on the lens barrel.)  Furthermore let's say this is a 
symmetrical lens so we won't have to figure the position of the entrance 
pupil separately.  

If you want to focus a 200 mm lens at 960 mm, then you have to rack the
lens out about 50 mm bringing it to about 250 mm from the film plane.
So the film has to move 65*(800+250)/800 = 85 mm.  Or you could say that
you shift the camera 85 mm and shift the lens back 10 mm towards the
center to make the lens only shift 65 mm.

John B


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