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Geometric optics and photography

  1. Aug 28, 2010 #1


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    Hi there,
    I have a question about photography. We know that in geometric optics, a bunch of parallel rays which going into the len will focus on the focus (a point). But as we see, the image is a set of points on a 2 dimensional plane. It is quite confusing that a focus is only a point instead of a plane. How can the len to catch the image of all points of the object?
  2. jcsd
  3. Aug 28, 2010 #2
    You said it.

    The diagram you speak of refers to rays coming from a single direction and are indeed focused to a point.

    However from the lens' point of view it sees light from the hemisphere in front of it. Rays through each point on the hemisphere can come from any direction. All rays parallel to any given direction are focused onto a plane.
  4. Aug 28, 2010 #3


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    oh I see. So you mean for a len, there are more than one focus? Only the one corresponding to a bunch a rays parallel to the principal axis locate on the axis. All others foci are on a plane. As shown in the picture, the green rays will be converged in the focus on the axis and other parallel rays making an angle with the green one (as shown in red) will be converged in the focus located on a plane that the principal focus on, is that right?

    [PLAIN]http://img820.imageshack.us/img820/9933/aaaqtk.jpg [Broken]

    Last edited by a moderator: May 4, 2017
  5. Aug 28, 2010 #4


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    Staff: Mentor

    Note that a bunch of incoming parallel rays corresponds to a point-object at infinity. In general, an ideal lens produces a point-image of a point-object. If you have an extended (non-point) object, you need to consider it as a collection of point-objects. From each point-object, the lens produces a point-image. The point-images taken together form the extended image which corresponds to the extended object.
  6. Aug 28, 2010 #5

    Andy Resnick

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    That is a good diagram. It's not that there is more than one focus, there is a focal *plane*.

    Objects not at infinity don't greatly change the diagram that much.

    A common aberration (field curvature) means the focal plane is actually a curved surface- that means your image (if you are using a flat detector) will not be in-focus everywhere. Your retina, which is curved, allows field curvature to be less problematic.
  7. Aug 28, 2010 #6


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    Gold Member

    You got it.

    The word is "lens" - singular.
    The plural of lens is "lenses".
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