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Thread: magnification of lens

  1. #1
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    Default magnification of lens

    Long ago somewhere in CS I was told that in order to find out the magnification power of a lens, I can simply take the picture of a ruler at the minimum focusing distance of the lens, and the length of the picture (i.e. the horizontal length reading of the ruler's image) as a ratio to 36 (actually the reciprocal) would be the magnification.

    e.g. if in the picture the ruler reading is 56mm then the lens has a magnification of 0.64 (36/56).

    Now for a 1:1 macro lens, that means the ability to take a ruler's picture in which the reading is 36mm, equal to the real size of the 35mm film.

    Am I correct so far?

    Now my question is: if the definition of 1:1 macro means that the image size is the same as the subject size, what about digital sensors? Most sensors are smaller than a 35mm film. So a 1:1 macro lens would produce an image of a 36mm object that is too big for the image frame. Yes?

  2. #2
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    A dSLR would produce a proportionately smaller picture at 1:1. A 1.6x dSLR should read 22.5mm on the same ruler at 1:1.

  3. #3
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    Default

    Quote Originally Posted by ST1100
    A dSLR would produce a proportionately smaller picture at 1:1. A 1.6x dSLR should read 22.5mm on the same ruler at 1:1.
    So so you think that for DSLR we basically factor in the cropping ratio? hence a true macro lens would produce a "apparently" higher magnification because the sensor could not accomodate the entire scene.

    So the definition of True Macro (1:1) for a DSLR would need to account for the cropping factor. Which means the method of snapping a ruler and reading the image "length" would not work for DSLR unless you know the cropping factor.

    Yes?

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