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A question of crop factor

Discussion in 'Nikon DX DSLR Forum' started by jamesd3rd, Jul 1, 2008.

  1. jamesd3rd


    May 4, 2007
    So. Cal
    A friend of mine and I got into a discussion of the DX sensor crop factor. Nikon lens specs will specify the 35mm equivalent as you well know. The question he had was when a non-DX lens is used on a DX body and the crop factor is implemented, are you getting the corresponding magnification that goes with that 35mm equivalent or are you just getting a cropped view?

    In other words, if you put a non-DX 300mm lens on a DX body and put that same 300mm lens on a FX body, placed both of them at the same position, does what I see in the DX viewfinder look larger compared to what I see in the FX finder due to the crop factor (since it is supposed to be equivalent to a 450mm on a FX body) or am I just seeing less with no increase in magnification?

    I hope I phrased that properly.
  2. jamesd3rd


    May 4, 2007
    So. Cal
    Surely someone knows

    18 views and no one has an answer?
  3. TonyBeach

    TonyBeach Guest

    You are seeing less of the image circle, which is why we refer to it as a cropped image. DX lenses project smaller image circles than FX lenses, but you get the same amount of magnification at a given focal length and focusing distance regardless of format. So a 30mm lens is always a 30mm lens, whether it is used on a DX sensor or an FX sensor and whether it projects a larger image circle or a smaller one.

    The magnification is the same regardless of format. Here's a simple way to think of it: Take a bee and place it on the sensor so that it is literally a 1:1 magnification. Now on a DX sensor you could fit one bee, but on an FX sensor you could fit nearly three of them, and on a 4x5 film negative you could fit a small swarm; but the magnification is always the same, it is 1:1.
    Last edited by a moderator: Jul 4, 2008
  4. dan1son


    Sep 24, 2007
    Another way to look at it is in relation to the field of view. Since the sensor size is different your field of view at a specific focal length is less. So they say the FOV of a 300mm lens on a 1.5x crop sensor is the same as that of a 450mm lens on a full-frame sensor (full-frame in this case is close to the same size as a frame of 35mm film).

    The only reason the idea of a crop factor came about was because film shooters knew what their view was out of specific lenses. Everything was in relation to the focal length on a 35mm film body. If you're used to what a 50mm lens looks like on a film body and all of a sudden you're using a DX digital body it made sense to say "75mm equivalent" so film shooters knew what to expect.

    Since I'm used to what lenses look like on DX bodies I have to do the opposite to know what it'll look like on a film or FF body. So a 50mm lens I'm used to on DX will look to me like a 33mm lens when the 50 is placed on an FX camera.
  5. pforsell


    Jan 15, 2008
    The latest Nikon DX cameras have higher viewfinder magnification than the FX cameras, if this is what you asked. Take D300 for example, in the viewfinder you see the 1.5x cropped area magnified 0.94x and in the D3 viewfinder you see the full FX area magnified to 0.7x. Both offer full 100% coverage of the respective image, a feature that is traditionally reserved only to the Nikon pro models.

    In the above example the D300 image is more magnified, but the total size of the viewfinder image is still larger on the D3 (about 15%) because the magnification difference is not 1.5x.

    The early Nikon consumer grade DX cameras (D70) had a viewfinder that could best be described as a "tunnel". Low magnification, small frame coverage and dark/dim ground glass.

    Here are simulated views through the viewfinders of D3 and D300 with same lens, same subject, same place. The D3 viewfinder image is a bit brighter due to the lower magnification. I hope I understood your question correctly.

    Subscribe to see EXIF info for this image (if available)

    View attachment 218948
    Last edited by a moderator: Jul 2, 2008
  6. Triggaaar


    Jun 15, 2008
    Your questions have been answered above, but I'd just like to see if I can help clarify this further.
    No, that is not the case. SLR lenses specify their focal length, full stop. You do get camera companies specifying the 35mm equivalent for compact cameras and the like, but not for SLR lenses. We often talk about 35mm equivalence, as do reviewers, as it helps us imagine the final image for different combinations of lens & body. And now that some of us are thinking of going for a D700, it helps us think what lenses we'll need to cover what we have now etc.
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