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  1. #1

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    Silly lens question

    Suppose you have two lenses; one is made of two parts cemented together and the other is of the same overall dimesions, but of one piece as shown below. Further, let's say the former is bound by a conventional cement.

    The question is: why do the two examples behave differently? Or do they? Is it the break made by the cement? If it is by the cement, then what if the cement had the same diffraction qualities of the glass?

    Or is something else happening, for example the light rays entering the first lens "have knowlege" of its boundaries an reconform to act differently when they hit the second?

  2. #2

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    In the two-piece example the elements are made of different types of glass possessing different optical properties.
    My Verito page

    Anyone can appreciate a fine print. But it takes a real photographer to appreciate a fine negative.

  3. #3

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    Quote Originally Posted by wfwhitaker
    In the two-piece example the elements are made of different types of glass possessing different optical properties.
    And if they were of the same glass?

  4. #4
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    Quote Originally Posted by jjstafford
    And if they were of the same glass?
    One would be easier and probably cost less to manufacture.

  5. #5

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    Quote Originally Posted by jjstafford
    And if they were of the same glass?
    Beats me. I'm not an optical engineer, nor do I play one on TV. My best guess is that two cemented elements of the same identical glass would behave as a simple lens. But maybe one more knowledgeable than I will reply.
    My Verito page

    Anyone can appreciate a fine print. But it takes a real photographer to appreciate a fine negative.

  6. #6
    rbarker's Avatar
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    I'm not an optical engineer, either, but Will's guess, given the (potentially artificial) parameters here, certainly sounds logical.
    [COLOR=SlateGray]"You can't depend on your eyes if your imagination is out of focus." -Mark Twain[/COLOR]

    Ralph Barker
    Rio Rancho, NM

  7. #7

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    Quote Originally Posted by jjstafford
    And if they were of the same glass?
    Actually, that's how Zeiss made center filters. Two pieces of glass of the same refractive index and dispersion, one plano-convex, the other plano-concave. The plano-convex one is thicker in the middle, is made of dark glass. The plano-concave is made of clear. Presto! Center filter.

    John, you have a history of asking stupid-seeming questions in order to be given the solution you want to a problem. What are you trying to accomplish this time?

    Cheers,

    Dan

  8. #8
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    "Actually, that's how Zeiss made center filters."

    Funny, but I always wondered how they made them. I fantasized a guy with an airbrush (or spray can) with gray paint pointing it at the centrer of a round piece of glass... :-)
    Now I know... But how do they make them so thin ? If they're made of two pieces of (not flat) glass, they should be thicker, shouldn't they ?

    Thank you, Dan.

  9. #9

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    Quote Originally Posted by Dan Fromm
    [...]
    John, you have a history of asking stupid-seeming questions in order to be given the solution you want to a problem. What are you trying to accomplish this time?

    Cheers,

    Dan
    Gosh, am I so transparent? Really, I'm simply curious about just this one point which I thought would be simple enough not to warrant spending all weekend in the library to figure out. Too much time on my hands right now; perhaps I'd best hit the books.

  10. #10
    Ole
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    If two clear pieces of glass with the same refractive index and dispersion are cemented together, the only difference over a single piece of glass would be more flare.

    At least that's my best guess...
    -- Ole Tjugen, Luddite Elitist
    Norway

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