RE: Narrowing the slit on OLD pan cameras

[Kurt Mottweiler <krm@xxxxxxxx>]

Marty,
  I understand the confusion but the issue is not one of depth of field. The
moving-film camera adds another layer of complexity.
  I once had occasion to run film backwards relative to image motion in a
linear panoramic camera called the Topocam. I was doing a series of tests
with different slit widths to answer one of many "what if" questions about
the camera. As you might imagine, shots made with wide slits produced images
with a significant amount of image "slur' for lack of a better term. But
with slit widths on the order of about 1mm a quite recognizable image
resulted. We played with the result conceptually and decided that if you
could recreate the image with in infinitely narrow slit width you would
likely achieve a perfectly clear image. (A sort of 'unwrapping' of the
reversed motion image onto the film.) The issue is akin to the kind of
motion blur you get when panning an SLR while shooting at modest exposures.
With a moving film camera you are trying to make the image stationary with
respect to the film. If your synchronization is not accurate you get an
image that appears not critically sharp. (BTW, the rear nodal point issue
fits right into this discussion)
  The point of all this is that the issue is motion-blurring of the image
rather than depth of field concerns. A narrow slit in a camera with
imperfectly synchronized image/film motion can 'correct' the image to a
limited extent. As to Andy's original point about wide angle lenses, I
believe he is referring to the possibility of differences in image motion
through the height if the slit that might result from using a very short
lens. That would seem to be another area where narrow slits would improve
the result with the caveat supplied by Alan regarding a downside of narrow
slit use - increased possibility of banding.
Kurt


>
>
> Alan -- I understand that if the film is not at the focal length
> the image
> will be blurred.  But please explain why narrowing the slit
> reduces the blur.
>  The shutter speed is faster, but the f/stop remains the same,
> therefore the
> depth of field (if that's the correct term in this situation)
> should be the
> same.  I'm having trouble getting past this concept, so please
> help. -- Marty
>