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Thread: Focal length confusion...

  1. #1
    Member benbean's Avatar
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    Default Focal length confusion...

    I gave up this hobby years ago and only recently awaken again...
    Suddenly the world of film is gone and welcome the digital era

    Now what is confusing to me is the 1.5x FOV vs image magnification.

    For example, a 50mm lense to me is same magnification what our eyes see, and a 200mm should be 4x magnification ratio. With digital, as always mentioned, 200mm should equate to 300mm in film. But 300mm to me is always a 6x magnification ratio..

    Am I the only one having problem with the conversion?

  2. #2
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    Default Re: Focal length confusion...

    Quote Originally Posted by benbean
    I gave up this hobby years ago and only recently awaken again...
    Suddenly the world of film is gone and welcome the digital era

    Now what is confusing to me is the 1.5x FOV vs image magnification.

    For example, a 50mm lense to me is same magnification what our eyes see, and a 200mm should be 4x magnification ratio. With digital, as always mentioned, 200mm should equate to 300mm in film. But 300mm to me is always a 6x magnification ratio..

    Am I the only one having problem with the conversion?
    quite right to say that 300mm is 6x compared to our eyes. but for if you compared it to a 50mm lens on a 1.5x FOV it will be 75mm and 300mm will still be 4x of 75mm. just like 200mm is 4x of 50mm.
    i think the problem lies with you took 1.5x FOV in to account for a 200mm lens and not 50mm lens thus you get 6x magification

  3. #3

    Default Re: Focal length confusion...

    Benbean, the problem is an artificial one. First, a 50mm lens is a 50mm lens, on film or on digital. The problem arises with the film/sensor format you are using and what you are doing later with the negative/file, how much you enlarge it.
    What makes the 50mm a "normal lense" for 35mm film is not the magnification factor but the perspective of achieved photo. A lense that has a focal length approximately equal to the diagonal of the negative/sensor produces a picture with similar perspective as what we see with our eyes. So for a medium format camera "normal" is about 80mm and for DSLR with APS size sensor normal is about 30mm and for the PS digital cameras its even less.
    However, a 50mm or 200mm lense will give you always the same magnification, provided you enlarge the photo to the same size and that means a photo taken by an APS senser will yield a slightly smaller positive than with a full frame (35mm) sensor.
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    Member benbean's Avatar
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    Default Re: Focal length confusion...

    hmmmm interesting answer Michael. The key word here is perspective.
    Your answer helps, now I just need to get accustomed to the new numbering...
    Thanks Bro.

  5. #5
    Member benbean's Avatar
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    Default Re: Focal length confusion...

    Thanks for the answer. When coming from old school of thought, it will take time to get accustomed to aps size sensor with 1.5x FOV.
    Cheers.
    Andy

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