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Thread: Calculating Focal Length

  1. #1

    Default Calculating Focal Length

    Hi, confused about focal lengths... if i was using a APS-C body,

    will there be a difference to the effective focal length when paired with my body? i.e, when two different lens are involved... 17-55mm designed for APS-C, and 17-55mm designed for full frame?

  2. #2

    Default Re: Calculating Focal Length

    No. The FOV crop refers to the magnification, or at least the illusion of one, achieved when you crop an image. Imagine an image that fills up your monitor. You crop the center of the image, delete everything outside of the crop, then use that crop to fill up the screen. That portion of the image is still the exact same as when it was still part of the main image, but now looks zoomed in.

    To apply that analogy to a DSLR, the original image is still there, it's the image circle formed by a FF spec'd lens. The crop happens because the sensor is smaller than the image circle, by about 1.5 times in a APS-C camera, so you get a zoomed in effect. The difference between a full-frame lens and a DX/DC/whatever lens is that the DX/DC/whatever lens leaves out the outside portion of the image circle, the part that doesn't hit the sensor and doesn't affect the sensor in any way.

    Because the digital crop lenses produce smaller image circles, most can't be used in full-frame or 35mm cameras without vignetting, created because the image circle isn't large enough to produce an image across the 35mm/FF frame.

  3. #3

    Default Re: Calculating Focal Length

    Quote Originally Posted by wlcling View Post
    Hi, confused about focal lengths... if i was using a APS-C body,

    will there be a difference to the effective focal length when paired with my body? i.e, when two different lens are involved... 17-55mm designed for APS-C, and 17-55mm designed for full frame?
    its there a 17-55 for FF? me kinda new here...

    btw...does it all matter?

  4. #4

    Default Re: Calculating Focal Length

    why want to calculate focal length?
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  5. #5
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    Default Re: Calculating Focal Length

    A useful piece of information to use with the max shuttle speed = 1/focal length formula? Otherwise I see little point.

  6. #6

    Default Re: Calculating Focal Length

    Quote Originally Posted by cantaresg View Post
    A useful piece of information to use with the max shuttle speed = 1/focal length formula? Otherwise I see little point.
    1/fl is only a guide, no point calculating... really.
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  7. #7

    Default Re: Calculating Focal Length

    shouldn't it be effective focal length, aka 35mm eq? focal length is a physical property of the lens itself?

  8. #8

    Default Re: Calculating Focal Length

    sorry... was getting confused earlier, especially when read reviews saying "this lens is equivalent to xx to xx on a full frame", etc...

    thanks for the explanations!

  9. #9

    Default Re: Calculating Focal Length

    when taking pics, how i noe wat apperture i going to use when i adjust my focal length. is there a way to noe?
    Good Person:think: With Lousy Camera -_-lll

  10. #10

    Default Re: Calculating Focal Length

    Quote Originally Posted by ray9211 View Post
    when taking pics, how i noe wat apperture i going to use when i adjust my focal length. is there a way to noe?
    depends on what you wanna achieve.. only you will know what you are going to use.. but in any case, theres a value in the viewfinder 2.8, 5.6 etc, thats your aperture value.. for most zoom lens, its a variable aperture, eg f/4-5.6, meaning the maximum aperture at the widest angle is f4 and at telephoto its f/5.6..

    hope this answers your question..

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