Converging fields of view

[P3D Neil Harrington <nharrington@xxxxxxxxxxxxx>]

Larry Berlin writes:

>I haven't had the pleasure of using a "stereo" camera 

Ah!  That explains that.    :-)

>so perhaps some of the
>stereo cameras do have non-parallel fields of view. 

Of the Realist-format stereo cameras I have the specs on, they _all_ do.

>If you are using a
>single camera with still objects you have to slide the camera sideways to
>obtain the second shot. If you do this very carefully with a slider bar, the
>"plane of coincidence", as Gregg calls it, is at infinity. Masking of an
>image will establish the "window" at some other depth.
>
>This topic is very device dependent and has very little meaning unless you
>are very specific about which device you are discussing and all the exact
>specifics of an individual circumstance. 

I think most people in the discussion have correctly assumed that the device
being discussed was and is the common Realist-format stereo camera.

>I checked a bunch of my stereo
>images and sure enough you appeared at first to be right, except for the
>fact that where the discrepancy you mention is visible in the full image
>frame, it only proves that I didn't maintain my camera at a parallel angle
>for each shot in the pair. 

Your comments have been, of course, entirely correct in the case of
slide-bar, single-camera stereo photography.  The overall fields of view for
both shots would be, or should be, perfectly parallel; there would be no
convergence or crossover.

You would still have to alter that by masking or cropping so that you
_would_ have converging fields of view, if you wanted to have a window
positioned somewhere in the stereo view.  No convergence, no window.  It
doesn't really matter in any way that I can see whether the masking is done
in the camera, as is the case with Realist-format cameras, or afterward.

>If such is true of images taken with a stereo
>camera, then it is not designed with parallel fields of view, or mounting
>has influenced where the "window" is located.

Or both.

>In a two camera shot, the center line of the lens axis matches the center
>line of the field of view. If the two cameras are maintained parallel to
>each other, and each lens has the same angle of total view, the left side
>camera will see more of the left field than the right side camera, all the
>way out to infinity. Your system may actually have some amount of
>"convergence" in it's fields of view, ie: non-parallel. If the aperture is
>offset from the center of the lens, it will result in the "converging"
>fields of view that you describe, and could still have a parallel lens axis. 

Yup.  That is exactly the case.  In a Realist-format camera the separation
of the film gates (center to center) is about 1 mm greater than lens
separation, which is what provides convergence in the fields of view.

>Full frame stereo images shot by two cameras, maintained exactly parallel,
>will not display the differences you describe. Images placed in mounts will
>display a "window" wherever the masking has set it. 

Again, all that means is that you are doing post-exposure what a
Realist-format camera does at exposure.


------------------------------