Does magnification increase when you use a FX lens on a DX camera?

Discussion in 'Nikon' started by anuragagnihotri, Aug 26, 2012.

  1. Hi,
    Many of you know that i'm collecting lenses for my D7000 and a future FX purchase.
    This is about a macro lens purchase, but might be true for all lenses...
    Lets say 60mm G macro has .15 meters MFD on a FX camera...when you mount it on a DX camera, the MFD remains same but you get a 1.5 crop factor...does this mean that it will go higher than 1:1, or conversely, does the minimum focussing distance increases for the 1:1 results? In simple terms, do you need to stand a little far than the stated MFD for 1:1 magnification?
    I am deciding between this, tamron 60/2 or 85 3.5 VR...
  2. The magnification stays the same only the things you can get 1:1 on a DX snesor are smaller. On a FX sensor they can be 36mm long(actually 43) and on a DX 24mm.
  3. Another way to think about this is that the properties of the sensor affect the image, but they do not in any way affect the intrinsic optical properties of the lens itself.

    The DX sensor in the D700 is 23.6mm x 15.6mm. An FX sensor is about 36mm x 24mm. A 1:1 magnification ratio results in a 10mm long object occupying 10mm of a DX sensor (38% of the 23.6mm width of the D7000 sensor) or the same 10mm on an FX sensor (where it is only 28% of the 36mm width of an FX sensor). If you then go ahead and print each photo without further cropping your 10mm object will occupy 38% of the width of the photo from the D7000 or 28% of the width of the photo from the FX sensor.

    The final photo ends up showing an object at different sizes in each case even though the real object is a fixed 10mm. Clearly there is a magnification step here, but this magnification comes from the ratio of the size of the sensor to the size of the photo. It's nothing to do with the lens.
    Printing a D7000 photo at 6x4 (roughly 150mm by 100mm) is a magnification of (150 / 23.6 =) roughly 6.4x. Printing an FX photo at 6x4 is a magnification ratio of (150 / 35 =) roughly 4.3x. These ratios don't depend on what lens was used on the cameras.
  4. 1:1 means an object that's 24mm wide fills the frame on DX and an object that's 36mm wide fills the frame on FX.
    Real world size translates directly to size on the sensor - but the size of the sensor is different.
  5. Sometimes we make this too hard...
    Seriously, you could just imagine cropping in on an image. That's what's happening. So the effective field of view goes up. No other lens properties actually change.
    A 300mm tele lens, used on DX, has the apparent field of view of a 450mm on FX or full-frame film. Focusing distance, f-stop, etc. do not change.
  6. There is also an effect with DoF. With FX, you lose one stop of DoF per f-stop. This is because DoF is controlled by (1) aperture (2) image size (or format size.) Thus, if you use a 100mm macro lens at f16 on a DX camera, it will have the same apparent DoF of f11 on an FX camera.
    Kent in SD
  7. That's the wrong way round Kent. The smaller format will show an apparent increase in depth-of-field for the same field-of-view, because the magnification from subject to sensor will be smaller. Image magnification has a far greater effect on D-o-F than the aperture number.
    Below is the same subject at the same aperture, shutter speed and ISO setting, under the same lighting and with the same lens-to-subject distance. The only thing that's changed is that the lefthand image was taken with a full-frame DSLR, and the right with a tiny sensor bridge camera. The difference between full-frame and DX isn't quite as obvious as this, but DX still has a D-o-F advantage - unless you want a shallow depth-of-field, and then it's a disadvantage.
  8. This could be a competition to see who can make it the simplest. Imagine the full frame sensor is instead a piece of 35mm film. Take your macro shot. A bug just fits in the frame, so the bug is about 36mm wide. Now trim off a few millimeters all the way around the piece of film. That's what you would get if you used a piece of aps film or an apx-c (dx) sized sensor, so now you've cut off part of the head and "tail" of the bug. It is still 1:1. If you want the whole bug in the frame with the dx sensor, move your tripod further away from the bug. Doing so will give you a little more depth of field at the same aperture simply because the camera is now further away from the bug. Also, now you won't be 1:1 anymore because you have made teh 36mm bug fit on a smaller width sensor, but you can still get 1:1 when you want. You just can't fit as much in the frame.
    So with the camera at the same distance from the bug, either dx or fx sensor gives you 1:1 ratio of image on the sensor, but the dx just shows less. It has a more narrow field of view when used at the same distance as the fx sensor
  9. You will get exactly the same size image in the center, but on the DX camera you will be "cropping" out the additional image area.
    So you don't actually get more magnification from the lens (in terms of resolution, acuity, etc.), but you get exactly the same effect if you cropped the center out of a larger image on an FX image.
    Thus the practical effect is the so-called "focal equivalence" (1.5X on Nikon) that never, ever should have been used in the first place. It's just a source of confusion for everybody who didn't start out as a wee chee-ild on 35mm film cameras. :(
    Technically, some APS-C (DX) camera sensors may have more pixels per cm than some FX sensors, and that case, you will actually get slightly higher resolution from the DX image out of the camera.
  10. i understand that lens properties don't change...and sensor size plays a role...
    But you still get a more magnified image with DX...isn't it?
    Lets say, i'm using a 180 2.8 on both formats, at minimum focussing distance:
    On FX: I will get a head and shoulder shot
    On DX: I will get a tight headshot
    When we print this picture 8x10, we see that the DX shot has a magnified view with the same lens.
    In macro, the same thing will happen...
    Weather its sensor crop, or whatever, but the fact is, i can get NEARER at MFD with the same lens on DX....
  11. "But you still get a more magnified image with DX...isn't it?" JDM has really answered your questions. SOME DX camera sensors can in effect give you the appearance of more magnification by providing you with higher resolution so you have the ability to crop your image more without loss of IQ. But it really depends on the sensors you care comparing.

    You need to learn and understand the difference between FOV (field of view) Vs focal length. A 180mm lens gives you 180mm on both DX and FX, but the FOV on DX is the equivalent on 270mm.
  12. The last sentence above should read "but the FOV on DX is the equivalent view of 270mm on a full frame camera."

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