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Thread: DX Lens on DX Body vs FX Lens on DX Body

  1. #1
    Senior Member loboclerk's Avatar
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    Default DX Lens on DX Body vs FX Lens on DX Body

    A quick noobie question here... can't seem to find the right words to search for, apologies if it had been answered before.

    Is a 30mm DX Lens on a 1.5x crop sensor effectively at 45mm? Or is it 30mm because it is a DX lens?

    Ultimately, is a 20mm FX lens going to be wider than a 30mm DX lens when used on a 1.5x crop sensor?

    Hope I'm making sense here.
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  2. #2
    Moderator daredevil123's Avatar
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    Default Re: DX Lens on DX Body vs FX Lens on DX Body

    Quote Originally Posted by loboclerk View Post
    A quick noobie question here... can't seem to find the right words to search for, apologies if it had been answered before.

    Is a 30mm DX Lens on a 1.5x crop sensor effectively at 45mm? Or is it 30mm because it is a DX lens?

    Ultimately, is a 20mm FX lens going to be wider than a 30mm DX lens when used on a 1.5x crop sensor?

    Hope I'm making sense here.
    A 30mm DX lens will forever be 30mm in focal length. A 30mm FX lens will forever be 30mm in focal length as well, irregardless if you mount it on a DX or FX cam.

    A view from a 30mm DX lens on a DX cam, will look the same as a view from a 30mm FX lens on a DX cam.

    focal length is forever constant. It will never change irregardless if on DX or FX body. It is the Field of View that changes.

    A 30mm DX lens on a DX cam will have a field of view of approximately 45mm (35mm equivalent)

    A 30mm FX lens on a DX cam will have a field of view of approximately 45mm (35mm equivalent)

    A 30mm FX lens on a FX cam will have a field of view of exactly 30mm (35mm equivalent = FF equavalent)

    HTH.
    Last edited by daredevil123; 23rd June 2010 at 07:56 PM.

  3. #3

    Default Re: DX Lens on DX Body vs FX Lens on DX Body

    Quote Originally Posted by loboclerk View Post
    A quick noobie question here... can't seem to find the right words to search for, apologies if it had been answered before.

    Is a 30mm DX Lens on a 1.5x crop sensor effectively at 45mm? Or is it 30mm because it is a DX lens?

    Ultimately, is a 20mm FX lens going to be wider than a 30mm DX lens when used on a 1.5x crop sensor?

    Hope I'm making sense here.
    Since daredevil has answered your qn, let me just add on abit

    A lens designated DX (or EF-S in Canon's case) has its image circle reduced to fit a crop sensor. So basically, if you try to mount a DX lens on a FX cam, you will see that the corners of the pic are black.

    FX lens on DX body is perfectly fine and you just use less of the image circle, focal length and aperture all same.

  4. #4
    Senior Member loboclerk's Avatar
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    Default Re: DX Lens on DX Body vs FX Lens on DX Body

    Thanks for the answers daredevil123 and brapodam!

    Had some friends arguing otherwise but it didn't sound right so I came here to ask. Thanks for the clarifications
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