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Thread: Qestion about Pentax-A 645 lens on a7rii

  1. #1
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    Mar 2010
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    Qestion about Pentax-A 645 lens on a7rii


    I bought a Pentax-A 645 55mm f/2.8 and a Pentax 645 to Canon EF adapter.

    The a7rii has a Fotodiox Pro EF to E-mount adapter on it.

    It all works fine.

    My question is about the field of view of the Pentax-A 55mm on the Sony. It's slighly less than what I see through my Sigma 50mm f/1.4 Art.

    Why are MF lenses said to be the equivalent to such-and-such a focal length (a wider angle) in 135 format.

    Shouldn't the Pentax-A 55mm be 'wider' than the Sigma 50mm?

    What am I missing?


  2. #2
    Senior Member thrice's Avatar
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    Aug 2009
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    Re: Qestion about Pentax-A 645 lens on a7rii

    Focal length is focal length. Field of view is determined by the image circle of the lens and the size of the recording medium (in this case the sensor).

    A 50mm on 645 is approximately equivalent a 28mm on 24x36mm (like your A7).
    A 50mm on 24x36mm is equivalent to a 50mm.

    If you had a condensor like one of the speed boosters you might get a wider perspective as the image circle would be condensed to fit on your smaller chip. That being said I don't believe a 645->135 speed booster exists.

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