

      The field of view for a camera lens shows from
      'Tools', 'Camera lens frame'.  The projection
      must be "Camera View".

      This tool allows simulations to be matched to
      halo photographs.  It also gives an impression
      of the size of halos in the sky.  

          You will find for example that a lens of 
          28 mm or shorter is needed to capture all 
          of a 22 deg halo.

      Set the lens focal length in the 'camera' box 
      at lower right.

      The default frame displayed is for approximately
      the shortest focal length lens which fits in the 
      visible simulation.***  

          The actual simulation field of view can be 
          changed from the control panel by opening
          the "advanced" section with the <-> button.  

          The zoom setting also changes the angular 
          size of the visible field.   Make the zoom
          box somewhat larger than the camera field 
          rectangle.

      Standard 35 mm lenses are shown.  Any other focal
      length can be entered in the box.

      Rotate the field clockwise or counterclockwise
      with the fast and slow buttons.

      The field rectangular cannot be moved.  A camera 
      field is non linear in angle and if the rectangle
      were moved it would no longer represent the true
      field of view.  Instead, move the centre of the 
      simulation using the Control Panel advanced 
      controls.

      Hide the field rectangle by closing the 'camera'
      box. 
 
      *** The shortest lens is 18 mm because the 
          simulation projection is for an ideal pinhole 
          lens.  Very short focal length lenses have 
          significant distortions.  You can enter one
          manually if you wish.



