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47271Re: [PanoToolsNG] Re: What is the actual Field of View?

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  • Erik Krause
    Jan 5, 2011
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      Am 05.01.2011 19:26, schrieb Michel Thoby:
      > I believe that for most (if not all) wide-angle lenses, the movement
      > of the entrance pupil is not basically due to the "selection of rays"
      > by lens components mountings that act as a secondary stops for
      > oblique ray bundles (as you seem to suggest). Even if the diaphragm
      > is closed down to let pass the narrowest possible ray and without
      > interception, the pupil shall moves. And with some lenses it may move
      > a lot! An animated illustration is shown for example on Fig.16 at:
      > http://www.pierretoscani.com/echo_fisheyes_english.html You may see
      > that the physical vignetting and the subsequent interception of rays
      > start to happen only at about 30° when the pupil had already moved
      > backward and away from the axis. BTW in a fisheye the same happens
      > but in a forward fashion.

      You are certainly right, although I believe that non-retro-focus
      (symmetrical) constructions, like they are used for large format, range
      finder and may be compact cameras, don't show the effect you and Pierre
      Toscani observed. I think it is inherent to retro-focus constructions only.

      However, since the location of the entrance pupil is constant for a
      given angle of incidence and doesn't change when stopping down, it
      doesn't affect the field of view.

      > The "normal" lateral distortion of a lens varies with the object
      > distance (and varies more by focus distance setting) but practically
      > not by very much. Whereas the movement of the pupil causes an
      > additional distortion that is negligible at large object distance
      > that becomes preponderant at close object distance and that compounds
      > the "normal" distortion. The height of the image is consequently
      > changed simultaneously by these factors. In short: distortion and FOV
      > depend on distance: the closer the distance is, the stronger is the
      > dependance. For a good illustration you may
      > see http://toothwalker.org/optics/distortion.html Paul van Walre did
      > not take explicitly into account the entrance pupil movement for
      > close range object imaging, but IMHO his photo clearly demonstrates
      > the resulting spectacular effect on figure 5!

      This is spectacular indeed. I'd assume that physical vignetting is
      involved. I don't have my EF 2.8/20 at hand at the moment to see whether
      it reacts the same and why. If this effect isn't caused by physical
      vignetting it should be possible to calculate the entrance pupil
      movement from the amount of distortion.

      But this leads to the question, what actually FoV is? Or better, where
      it is measured. If we define FoV as the angle between the outmost single
      light rays just hitting the sensor, there is nothing like a distance.
      You always need a fan of rays (or at least two) to define a distance.

      I admit that this is a rather academical point of view. For panorama
      creation the outmost points in the image are of interest. And indeed the
      angle in object space corresponding to those points may differ depending
      on object distance - be it only because one side of the entrance pupil
      is nearer to the object than the other one.

      Erik Krause
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