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Thread: What kind of lens bends an image without distortion?

  1. #1

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    What kind of lens bends an image without distortion?

    Would it be a simple trapezoid with flat faces?

  2. #2

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    Re: What kind of lens bends an image without distortion?

    I don't think so.

  3. #3

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    Re: What kind of lens bends an image without distortion?

    What do you mean by "bends an image without distortion"? Surely if anything is bent it's distorted.
    Rob Gray — Nature Photographer Extraordinaire
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  4. #4

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    Re: What kind of lens bends an image without distortion?

    Quote Originally Posted by GRAYnomad View Post
    What do you mean by "bends an image without distortion"? Surely if anything is bent it's distorted.
    I mean, for example, ability to look around a corner. But without mirrors, just a lens that bends the image 90 deg.

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    Re: What kind of lens bends an image without distortion?

    Quote Originally Posted by johnywhy View Post
    I mean, for example, ability to look around a corner. But without mirrors, just a lens that bends the image 90 deg.
    So a lens that bends light. That would need a gravity field similar in size to that produced by a black hole would it not?
    Rob Gray — Nature Photographer Extraordinaire
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  6. #6

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    Re: What kind of lens bends an image without distortion?

    porro prism ... from binoculars

    But maybe you wouldn't care about left to right reversal(?)

  7. #7

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    Re: What kind of lens bends an image without distortion?

    A bundle of optical fibres would quite literally bend around a corner. But it isn't a lens.
    Expert in non-working solutions.

  8. #8

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    Re: What kind of lens bends an image without distortion?

    Hi!

    ... a simple trapezoid with flat faces?

    ... is named: a prism.
    https://en.wikipedia.org/wiki/Prism

    An yes, a prism, working in transmission mode, is able to bend light rays; but bending by 90° is very difficult with a prism used in transmission mode. For example a thick prism with an angle of 60° and a refractive index of 1.5 will bend rays by about 45°.
    Prisms are not used in transmission mode in photographic lenses, with the important exception of anamorphic lenses used in taking and projection lenses for movie pictures, this is another story.

    Prisms used in transmission mode have the fundamental property to bend light rays by a different angle of deviation for different colors. Hence this property is really catastrophic in order to get sharp color images! Moreover it can be shown that a divergent or convergent beam deviated by a prism is severely affected by an aberration phenomenon which, again, will severly blur the image, even if the image is taken in monochromatic light.

    For all above mentioned reasons, when you want to deflect a beam by 90° and preserve the image quality, reflections on flat surfaces are preferred.
    Either flat mirrors (optical "first surface" mirrors, since bathroom mirrors with their coating at the back are not used in optics!) or ... prims used in reflection mode, like in pentaprisms for reflex cameras.
    A flat reflecting mirror, unlike a prism used in transmission, induces no aberration at all; all colors are reflected at the same angle and a converging beam will focus after reflection exactly like if there was no mirror at all (providing that the mirror is really flat! Again, we are speaking about precision optical mirrors), except that the mean ray of the beam will be bent by 90° when the mirror angle is 45° with respect to the mean beam ray.

    Internal reflections inside a prism are preferred when a combination of reflections is needed, like in the pentaprism.
    The first reason is that a well-machined and well-polished piece of glass will keep the relative angles of the different reflecting surfaces forever and does not need a complex mounting / glueing procedure like with a combination of simple plane mirrors. Moreover, a well-machined pentaprism will not exhibit any image splitting: with a combination of individual mirrors, it is very difficult to join them so precisely that no image splitting effect is visible at the junction between two mirrors.

  9. #9

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    Re: What kind of lens bends an image without distortion?

    Quote Originally Posted by Havoc View Post
    A bundle of optical fibres would quite literally bend around a corner. But it isn't a lens.
    It's still optical. Great answer!

    Quote Originally Posted by Emmanuel BIGLER View Post
    ... a simple trapezoid with flat faces?
    ... is named: a prism. An yes, a prism, working in transmission mode, is able to bend light rays; but bending by 90° is very difficult with a prism used in transmission mode. Prisms used in transmission mode have the fundamental property to bend light rays by a different angle of deviation for different colors. Hence this property is really catastrophic in order to get sharp color images! ...flat surfaces are preferred. Either flat mirrors or ... prims used in reflection mode
    Awesome answer! Yes, a pentaprism is what i'm thinking of.

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    To be more specific about my use-case:

    If my camera position is fixed and unchangeable, but it's not pointing directly at my subject, then i want an adjustable optical piece that i can manually tilt in order to redirect the off-access image into the camera. It would likely be less than 45 degrees, with a relatively small adjustable range, say +-20 degrees.

    i'm thinking something like a pentaprism would be the simplest solution. But, as can seen from the images above, the image could be reverse or upside-down, so it's a matter of figuring out the correct polygon of the prism to ensure that the camera sees a correctly-oriented image. That's why i'm wondering if a simple trapezoid would do. It might need to be a double trap, to correct the inversion.

    Or, why not just a rectangular prism? Just a simple square block of glass.
    Attached Thumbnails Attached Thumbnails 40x6x6mm-Optical-Glass-Dove-Prism-K9-Trapezoidal-Prism-Lens-Medicine.jpg  

  10. #10
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    Re: What kind of lens bends an image without distortion?

    Sinar used to offer a front-surface mirror (catalog no. 538.11) that could be mounted in a Sinar 4x5 standard and positioned on the rail in front of the lens for just this purpose.

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