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Type of image from an afocal optical system

  1. Jan 31, 2012 #1
    Hello Forum,

    what type of image does an afocal system produce if the object is an extended object? A real or virtual image?
    Any example? Say a car is located 30 meters away.....
  2. jcsd
  3. Jan 31, 2012 #2
    I just found a book chapter, AFOCAL SYSTEMS, by William B. Wetherell....

    It seems that afocal system can form real images...but how, if the emerging rays are parallel?

  4. Jan 31, 2012 #3
    I guess this systems always imply that the human eye is there....that is the only way an image will form: each point on the object will have parallel rays emerging from the system.
    The eye optical system will focus those bundles of rays on the retina and form a real image....
  5. Jan 31, 2012 #4

    Andy Resnick

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    The only afocal systems I am familiar with are telescopes; an eyepiece is required to form an image.
  6. Feb 4, 2012 #5
    Well, I was wrong in what I said.

    If the object is very far, the input rays are parallel and the output rays are parallel too.
    But if the object is close by, the rays entering the afocal system are diverging. Real images are possible. Moving the object along the axis does not change the transverse magnification, i.e. the image size...

    See figure 1D at http://www.mntp.pitt.edu/Workshop/MNTP_Prtcp_res_2010/teaching/Optics_Chapter_95_LanniKeller.pdf [Broken]

    Last edited by a moderator: May 5, 2017
  7. Feb 21, 2012 #6
    An image from afocal system always is real. 2 types afocal; Keppler with 2 positive lens and Lagruerre with 1 positive len + 1 negative len, but f of positive always > f of negative.
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