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What is the difference between, for example, a Nikon 50mm DX lens and a Nikon 50 mm FX lens? Both have the same focal length of 50 mm, but why does Nikon produce a DX lens and an FX lens? What is the difference in the lens construction? Is it just the size of the lenses is smaller in DX compared to FX because FX has a larger sensor? Any other differences apart from size, price and weight?

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Price, weight and size, but in particular, it seems important that the DX lens projects a smaller image circle, large enough to cover the DX sensor, but it is not designed to fully cover the larger FX sensor. It can vary with zoom in individual lenses, but generally, large areas in the FX frame corners are dark with a DX lens. A reasonable simulation of this is shown at https://photographylife.com/using-nikon-dx-lenses-on-fx-cameras

  • Thank you. The link you have shared was very useful as it had helped me understand the difference better. – seeknew Nov 18 '19 at 13:34
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    The FX lens can of course be used on DX without any issue, but the DX lens won't fully cover a FX sensor. Not asked, but one very common confusion about this topic is that there is no conversion of "Equivalent Focal Length" then, because any 50 mm lens always remains a 50 mm lens on either sensor. Equivalence is only a matter of the field of view that the sensor size sees with that 50 mm lens. A small sensor can "crop" that Field of View seen. Meaning, 50 mm on DX sees a Field of View that an "Equivalent" 75 mm lens would see on FX, but a 50 mm lens obviously remains 50 mm on any sensor. – WayneF Nov 18 '19 at 15:32
  • Thanks for this. I have read lot of other posts about the "equivalent focal length" but your explanation here nails it. – seeknew Nov 18 '19 at 16:29
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Since a DX sensor is smaller, it is possible to cover the sensor area with a smaller lens even at the same focal length and maximum aperture. A smaller lens is a lighter, cheaper lens too, so DX lenses provide benefits for both the vendor and consumers.

  • Thank you. This was my understanding too. – seeknew Nov 18 '19 at 1:08

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