Crop factor = ? x lens mm ?

Discussion in 'General Technical Discussion' started by Elf_8, Nov 20, 2005.

  1. I have it understood that the DX captor brings a 1.5x factor applied to the framing of the equivalent in mm of lenses rated in terms of 35mm film.
    Doesn't sound that clear...
    That is a 50mm lens will render an image in a frame equivalent to that of a 75 mm.
    I would like to know what is the math behind that could also be applied to calculate the transformation when applying a crop to an existing image. I would like to express that in terms of pixels.
    I'm trying to figure out how this would translate if I would crop a 10.2 mp image from the coming D200 to one that has 6.1, like the D100 or D70 have.
    From Tom Hogan's site, I read concerning the D2x that it has: " A high-speed crop mode with a reduced angle of view of 2x from full frame (as opposed to the usual 1.5x) and reduced number of pixels (6.9mp from 12.2)."
    6.9/12.2 = 0.57. Is it this .57 that is added to the 1.5 x factor to attain the 2x or so ?
    If I will extract a 6.1 mp image from a 10.2 delivered in the D200, it would bring an additional factor of 6.1/10.2 = 0.6, thus having the framing factor of 2.1x.
    That would mean that a 300mm shot in 10.2 mp but cropped to about 6 (with a resolution equal to that of the D100 or D70) would give a framing close to that of a 600 mm on a standard 35mm camera.

    Is this pure nonsense or what ?
  2. damnifino
  3. just kidding around with you Christian as my head was spinning after reading your post. I think your logic is sound, however.
  4. Christian, your reasoning is right, but only for angle of view and magnification.

    The crop factor due to smaller size capture devices or induced by the reduction of the image's size (10,2 Mp to 6 Mp) narrows the field of view and lends to an «apparent» magnification (just think about the electronic zooming feature in some P&S cameras), that in turn is «translated» into a virtual increase in focal length.

    This translation process is only valid for magnification. For other matters like DOF and perspective, everything remains the same since the real focal length doesn't change.
  5. I agree Dang, that's why I talked about "framing" equivalent, and not optical. For example, obviously, cropping cannot change the look of the background (DOF and perspective). That said, is the math OK ?
  6. perhaps it would be better do do the calculations based on linear rather than square resolution?
    i.e. 3216 horizontal ocmpared to 4288 horizontal- HSC vs DX for D2X- 0.75 factor converting 4288 to 3216. Identical factor 2848 to 2136 for vertical resolution
    D200 is 3872 horizontal pixels, 2592 vertical. Therefore a 2x FOV crop from 35mm in post, a la D2X HSC except done afterwards (though admittedly this serves no practical purpose other than framing, considering no speed increase or filesize out of camera change), would be 2904 by 1944- ever so slightly less than the D1X...5.6 megapixels.
  7. Interestingly, if you take .75 horizontal times .75 vertical, you get .56 which applied to the 10.2 mp (a square value) brings you to 5.7 mp.
  8. linear resolution will give you a more precise figure, and the exact dimensions/aspect ratio though.
  9. Right, x by y is what it takes to get the exact figure.
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