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Thread: reproduction ratio and crop factor

  1. #1
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    Default reproduction ratio and crop factor

    There's something confusing about reproduction ratios and sensor size crop factor.

    For example, some lenses states 1:2 reproduction ratio, this means the image formed on the negative will be half the size of the actual object (2cm object becomes 1cm on the film).

    With DSLRs, the introduction for crop factors has confused me a bit. Taking a 1.5X crop factor (for easy math) does that mean that the object is 'apparently' 1.5X bigger? Even though the actual image formed on the sensor is the same as a full frame sensor (or negative)

    Am I right to say for 1:2 lenses it now appears to be 1:1.3?

    I'm applying the same theory of the focal length where 100mm is now appearantly 150mm.

    so 1:1 lenses are now "1.5:1"
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    Default Re: reproduction ratio and crop factor

    Since the reproduction ratio is the ratio between the image size on the recording medium (film or CCD) and the actual size of the subject, it does not change regardless of the size of the recording medium.

    Say you have a 1:1 lens and use it to shoot the same subject on film then on a DSLR with 1.5x crop factor. The size of the image on film is the same as the size of the image on the CCD. If you print the images from both cameras to the same size print, say 8X10, the magnification from the DSLR will be larger, so the object will be larger on the print from the DSLR.

    Does it make things clearer if you were to visualize shooting 1:1 on 35mm film and then you take the film and cut away the outer edge to get a 1.5 crop factor? The size of the image on the film does not change right?
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    Default Re: reproduction ratio and crop factor

    Quote Originally Posted by roygoh
    Since the reproduction ratio is the ratio between the image size on the recording medium (film or CCD) and the actual size of the subject, it does not change regardless of the size of the recording medium.

    Say you have a 1:1 lens and use it to shoot the same subject on film then on a DSLR with 1.5x crop factor. The size of the image on film is the same as the size of the image on the CCD. If you print the images from both cameras to the same size print, say 8X10, the magnification from the DSLR will be larger, so the object will be larger on the print from the DSLR.

    Does it make things clearer if you were to visualize shooting 1:1 on 35mm film and then you take the film and cut away the outer edge to get a 1.5 crop factor? The size of the image on the film does not change right?
    Well said.... I like the part where you mention take the film and cut away the outer edge... very useful for visualization....

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    Default Re: reproduction ratio and crop factor

    Quote Originally Posted by roygoh
    If you print the images from both cameras to the same size print, say 8X10, the magnification from the DSLR will be larger, so the object will be larger on the print from the DSLR.
    this is the part that got me confused. I understand that the image formed on the sensor is of the same size regardless of full frame or "cropped frame"

    but it is becos of the cropping ("cutting away the negative") that made me wonder if it is the same effect of having a higher mag ratio on a full frame.

    i think i'm a bit clearer now. thanks roy.
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    Default Re: reproduction ratio and crop factor

    Quote Originally Posted by yanyewkay
    but it is becos of the cropping ("cutting away the negative") that made me wonder if it is the same effect of having a higher mag ratio on a full frame.
    The magnification of the print with respect to the object is determined by the magnification of the print wrt the negative/sensor, and the magnification of the negative/sensor image wrt to the object: M_print_object = M_print_sensor * M_sensor_object. M_sensor_object is determined by the lens and indepent of the film/sensor size (the optics don't know or care about it). M_print_sensor is determined only at the time you make a print, and independent of the lens used for taking the photo.

    The magnification of the lens is thus independent of the film/sensor format, but M_print_sensor will be larger if a small sensor/negative gets enlarged to a certain print size. Thus, the total magnification is affected by the sensor/negative format.

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    Default Re: reproduction ratio and crop factor

    Quote Originally Posted by yanyewkay
    this is the part that got me confused. I understand that the image formed on the sensor is of the same size regardless of full frame or "cropped frame"

    but it is becos of the cropping ("cutting away the negative") that made me wonder if it is the same effect of having a higher mag ratio on a full frame.

    i think i'm a bit clearer now. thanks roy.

    The difference in magnification occurs when the image is transferred from the recording medium to the print.

    Just some rough estimates here, because I don't have the exact measurements, but the principle is the same:

    For the original negative, to print at 8X10, maybe the magnification is 20X. Now for the cut film, since it is now smaller than the original full-frame film, to print at 8X10 the magnification required is then 30X. Since the size of the image of the subject remains the same when the film is cut, the print made from the cut film will show a bigger image of the subject.

    I will make a diagram to demonstrate this later.
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    Default Re: reproduction ratio and crop factor

    Here you go. Hope this makes it clear.

    If you have a 1:1 lens and captured the same subject at 1:1 onto film and then onto CCD with 1.5x crop factor, the size of image of subject on the film is the same as that on the CCD. However, if both captures are blown up to 8R, assuming no further cropping of the digital image captured from CCD, then the CCD image is magnified more times then the film image to get to 8R, thus the subject appears larger.

    As complexity rises, precise statements lose meaning and meaningful statements lose precision.

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    Default Re: reproduction ratio and crop factor

    wow.. thanks roy.. very very clear

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    Default Re: reproduction ratio and crop factor

    very clear explanation....
    see pic understand everything liao....
    nice....
    Hope to learn from everyone here....

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