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Thread: Of focal length, crop factor & EXIF

  1. #1
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    Default Of focal length, crop factor & EXIF

    Using a 50mm lens on a Nikon D50, the effective focal length is 75mm (due to 1.5x crop factor). However I noticed that the EXIF still shows 50mm instead of 75mm.

    Does that mean, we have to always keep in mind the crop factor when choosing a new lens for any DSLR? In addition, to find the actual focal length used, we will need to take from the EXIF, and multiply with the crop factor?

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    Default Re: Of focal length, crop factor & EXIF

    the crop factor does not increase focal length of a lens.

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    Default Re: Of focal length, crop factor & EXIF

    Just take the crop factor as a simulated focal length.

    for 35mm : Focal length is the focal length stated.

    for APS sensors : Simulated focal length is the crop factor X the focal length.

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    Default Re: Of focal length, crop factor & EXIF

    So when taking pictures using APS, does the lens' FL or the simulated FL count?

    Quote Originally Posted by TMC
    Just take the crop factor as a simulated focal length.

    for 35mm : Focal length is the focal length stated.

    for APS sensors : Simulated focal length is the crop factor X the focal length.

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    Default Re: Of focal length, crop factor & EXIF

    Crop factor is just a convenient way to determine the angle of view with lenses on a digital camera with a sensor size smaller than a 35mm film. In actual fact, the focal length of the lens does not change but the image captured can be likened to cropping out the centre part of the 35mm film. By doing this, you will lose the original angle of view but only have a smaller angle of view. Therefore, say, an 18mm lens which would have give an angle of view of 100 degrees on a 35mm film would only give an angle of view of only 76 degrees on a APS size DX format DSLR. This angle of view is what a 27mm lens would give you if used on a 35mm film body. There is no magic to it. Hope this is able to clear the confusion.

    However, I do hope Nikon could report to the external flash the factored focal length if the flash is used on a digital body. Otherwise, the flash is illuminating more area than necessary.

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    Default Re: Of focal length, crop factor & EXIF

    focal length properties of the lense remains unchanged be it shooting film or digital. the crop factor comes in because of the smaller sensor size, relative to 35mm full-frame. this link gives a pretty good explanation of the magnification factor -

    http://www.luminous-landscape.com/tu...dslr-mag.shtml

    [read link first]... not trying to confuse your even more, but the magnification factor is also different from the crop factor. e.g. Oly DSLRs (4/3 system) are full-frame (1.0x crop factor) but 2x magnification. that is because the image circle from the digial lenses fit nicely around the sensor, but the sensor is only 1/2 the size film FF (thus 2x mag).

    short answer to your question - the EXIF data would only state the focal length of the lense. so in choosing an appropriate lense for your shoot, you would have to taken into account the magnification factor in choosing a lense with the appropriate range.

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    Default Re: Of focal length, crop factor & EXIF

    EXIF data states the acutal focal length of the lens... the lens reports its focal length to the camera, which saves it in the EXIF data. The camera doesn't modify the information the lens transmits.

    If you want the effective focal length you gotta do the math yrself.
    Last edited by solarii; 8th January 2006 at 01:18 AM.

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    Thumbs up Re: Of focal length, crop factor & EXIF

    Thanks bros, for the explainations and link! I understand that the EXIF info is taken directly from the lens, so it's a true reflection of the focal length the lens was set to. Just that the DSLR crops the picture, hence the term "crop factor".

  9. #9

    Default Re: Of focal length, crop factor & EXIF

    Quote Originally Posted by huggable
    Thanks bros, for the explainations and link! I understand that the EXIF info is taken directly from the lens, so it's a true reflection of the focal length the lens was set to. Just that the DSLR crops the picture, hence the term "crop factor".
    That's why APS-sized sensor DSLR camera users need to use really very wide angle lenses (e.g. 14mm or 17mm) if they want "wide" angle because using a 24mm lens on a APS-sized sensor would be like using a 36mm-38mm lens on a 35mm full frame size.
    Last edited by Clockunder; 8th January 2006 at 01:56 AM.

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    Default Re: Of focal length, crop factor & EXIF

    Quote Originally Posted by huggable
    Thanks bros, for the explainations and link! I understand that the EXIF info is taken directly from the lens, so it's a true reflection of the focal length the lens was set to. Just that the DSLR crops the picture, hence the term "crop factor".
    i just noticed in the Opanda IExif 2.25, there are 2 entries referring to focal length,

    "Focal Length", e.g. 35mm (taken from the lense)

    "Focal Length In 35mm Film", e.g. 52mm (taking into account magnification factor).

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