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Thread: Prism Representation of Lenses

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    Prism Representation of Lenses

    A plus spherical lens is often represented by two prisms base to base. A prism will redirect an image but not refocus it, while a spherical lens will refocus it. When looking through, say, only the bottom portion of a spherical lens (the area below the optial center), it seems that you would be looking through essentially a base up prism. Yet the image still gets "corrected" while looking through a simple base up prism, the image is not corrected. What am I missing? Can spherical lenses really be thought of as prism combinations? Thanks for any insight on this.

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    Of course the spherical lens still has the focusing power in it. The prism is just a by-product of the unequal thickness of the spherical lens.

    Two prisms base to base (or end to end) would have flat, non-parallel surfaces, which will "move" an image, but not refocus. An actual spherical lens can be approximated by "prisms" w/ a CURVED front and back surface. The difference in the curverature (front to back) is what changes the focus. The difference in the thickness (again front to back) is what provides the prism.

    I hope this didn't confuse the issue even more.

    If you draw the pic of two prisms apex to apex - you have a plano lens that will redirect the image. Now draw a curve on the front and back surfaces of the prism. Now you have a refocusing prism. The prism will redirect the image and the curves will refocus it.

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    Yes, that helps. I wonder if you took two large regular prisms joined base to base, and looked through the pair directly through the junction, whether any "correction" would occur. You would be viewing half of an object through the lower prism and half the object through the upper prism. Therefore, it seems that the pair might actually be acting like a curved lens. Maybe my confusion is that I was thinking about viewing relative only to the optical center of the lens, when in fact any point you look through defines where these "representative" prisms are. In other words, I was picturing the typical prism to prism representation of a lens like you would see in any text, then imagining looking through say the bottom half only. I was thinking that by this representation you would be looking through only one prism now. But wherever the view is (whether through the OC or not), defines where this prism to prism relation can be thought of as being. This is something that I just got to thinking about and find very interesting. Thanks.

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    Quote Originally Posted by go_hercules View Post
    Yes, that helps. I wonder if you took two large regular prisms joined base to base, and looked through the pair directly through the junction, whether any "correction" would occur. You would be viewing half of an object through the lower prism and half the object through the upper prism. Therefore, it seems that the pair might actually be acting like a curved lens. Maybe my confusion is that I was thinking about viewing relative only to the optical center of the lens, when in fact any point you look through defines where these "representative" prisms are. In other words, I was picturing the typical prism to prism representation of a lens like you would see in any text, then imagining looking through say the bottom half only. I was thinking that by this representation you would be looking through only one prism now. But wherever the view is (whether through the OC or not), defines where this prism to prism relation can be thought of as being. This is something that I just got to thinking about and find very interesting. Thanks.
    You would get a correction. It wouls be essentually a very heavy cylindrical lens. To get a sphereical correction you'd need to have hundreds of piecies of prisms placed around in a circle.

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    Yeah, I think you're right on that one. I think maybe what I was describing might even act like a Maddox rod since it would just be stacked prisms... Thanks for the info.

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    The missing piece to the puzzale here is that you would need stronger and stronger prisms as you deviated from the OC. Two prisms base to base just represents the lens in a crude fashion. Look at the images below for an example:
    Attached Thumbnails Attached Thumbnails lens.PNG   prism.PNG  
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    Thanks for that info, it really helps my understanding. Thanks to everyone else too that helped me out.

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