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Thread: Wide-angle lens focusing

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    Wide-angle lens focusing

    The distance between the mount and the sensor is 40-50mm and the mirror takes up a lot of space as well. Then how does a super wide angle lens focus at infinity?

  2. #2
    RustBeltRaw's Avatar
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    Re: Wide-angle lens focusing

    A lens's focus distance isn't limited by the mount-to-film/sensor plane distance.

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    xpatUSA's Avatar
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    Re: Wide-angle lens focusing

    Quote Originally Posted by RustBeltRaw View Post
    A lens's focus distance isn't limited by the mount-to-film/sensor plane distance.
    Indeed. Rays originating from infinity come into the lens as parallel lines. At infinite focus, a simple single lens would need to be, as you suggest, a shorter distance away than mount-to-sensor distance. But the lens groups are arranged inside a wide-angle lens body so that that doesn't happen. How it works, I don't know. Some will.

    The opposite question would be about a telephoto lens, if we think about it.

    SagnikB, You got a name?

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    GrahamS's Avatar
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    Re: Wide-angle lens focusing

    Modern wideangle lenses, especially those designed for a DSLR, are of "inverted telephoto" design, resulting in the flange focal distance being much greater than the nominal focal length.

    http://en.wikipedia.org/wiki/Ang%C3%A9nieux_retrofocus

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    rpcrowe's Avatar
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    Re: Wide-angle lens focusing

    Although today we often call all lenses of long focal length, telephoto lenses, that is actually incorrect in the original usage of the term, "telephoto". Telephoto lenses were those lenses, usually of long focal length, in which the distance between the optical center of the lens and the film (or sensor) plane was shorter than the focal length of the lens when that lens was focused at infinity.

    As an example, a 400mm telephoto lens, when focused at infinity would have the distance between the optical center of the lens and the film/sensor at 400mm (about 16 inches). The lens which has the lens to focus plane distance at infinity the same as the focal length of the lens was correctly called a "long-focus or long-focal length lens). This could definitely result in some pretty physically large lenses...

    The wide angle lens works like the telephoto but in reverse. The focusing distance at infinity is longer than the focal length of the lens.

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