8cf1dd51   
   From: boyd@apt0.sao.arizona.edu   
      
   Speedy wrote:   
   > Hello,   
   >   
   > When I look at an object through a lens what is the apparent distance   
   > of that object when both the observer and the object are closer to the   
   > lens than its focal length?   
   >   
   > Background: I would like to focus a camera system at a particular   
   > distance. However, our lab is not large enough to cover that distance.   
   > Rather, I would like to place the camera inside a rack where it looks   
   > at a target and then focus on that target. Since the target is much   
   > closer than the distance I would like to focus the camera on I would   
   > place a lense between the camera and the target. For example, this   
   > could be a lens with 1m focal length. The lens is placed about 10-15cm   
   > above the camera and the target is placed behind the lens at some   
   > distance.   
   >   
   > How can I compute where the target should be placed so that to the   
   > camera it appears it be at the desired distance?   
   >   
   > Thanks in advance for any help!   
   >   
   > Regards,   
   > Marcus   
      
   You cannot simulate exactly an object which is at a further distance   
   than it actually is from the camera if the object is large compared to   
   the aperture of the camera lens and the actual distance is limited.  The   
   perspective to the object will be wrong.  Even with a large aperture   
   lens  you need a telecentric lens design  to get a more distant   
   effective perspective.   If you simply want the image to be smaller on   
   the film (or sensor) just use a shorter focal length lens, but that will   
   not give the same perspective as having the object placed at a further   
   distance.   
      
   You can use a large (at least half as large as the object )  flat mirror   
   in the room to double the optical  distance from the camera to the   
   object.  Or use two mirrors to triple the distance.  With an even number   
   of mirrors the object will regain it's left-right hand appearance.   
   However, the mirror nearest the target  will need to be over 2/3 the   
   linear size of the target for two mirrors, or 4/5's for four mirrors   
   give an effective five times the distance.   
      
   You could use your camera and existing lens to act as a large   
   telecentric lens by moving the camera in in an X-Y grid (z being toward   
   the object)  pattern then tiling the centers (or offsets from the center   
   depending on the desired perspective)  of each image together to form a   
   new image with an effectively longer (even infinite) perspective.  A   
   10x10 array of images would probably be sufficient so that the edge   
   distortions of each image would not be detectable or at least not   
   noticeable.  It would be tedious work to do manually.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)