Depth of Field Math

Discussion in 'Photography Beginners' Forum' started by GTHill, Oct 14, 2008.

  1. GTHill

    GTHill TPF Noob!

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    I have been having problems with picture focus. I am using a f4.5 300mm lens on my D300. Here is what I would like to see. How do I compare the DoF of a 200mm 2.8 Lens vs my 300mm 4.5 Lens? I think my DoF is too shallow because I'm not compensating for the extra focal length compared to the 200mm lens.

    So, in the end, how do I compare f-stop vs focal length when figuring DoF? I hope this makes sense...

    Thanks!

    Gene
     
  2. astrostu

    astrostu I shoot for the stars

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    I don't have my optics book in front of me because I'm at a conference. However, Wikipedia has what looks like a pretty thorough explanation along with equations here: http://en.wikipedia.org/wiki/Depth_of_field .
     
  3. Helen B

    Helen B TPF Noob!

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    The formula for non-close-up distances given in the Wikipedia article can be further simplified to give the approximate relationship

    DoF = 2 N c s^2 / f^2

    where
    N is the f-number
    c is the maximum acceptable circle of confusion
    s is the subject distance, and
    f is the focal length of the lens.

    The same units should be used throughout.

    This doesn't apply at near distances or if the subject distance is close to the hyperfocal distance. It shows that in the range that it does apply the DoF is the same for the same subject magnification at the same aperture, irrespective of the focal length of the lens (N and c are constant, f and s vary in proportion with one another, so their ratio remains the same).

    Best,
    Helen
     
  4. Shine

    Shine TPF Noob!

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    Helen, YOU ROCK!!!!!!! :hail:
     
  5. Steph

    Steph No longer a newbie, moving up!

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    Alternatively, if you can't be bothered with the maths, you can use this online DOF calculator: CLICK.
     
  6. GTHill

    GTHill TPF Noob!

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    Thank you very much for the math help. It did confirm what I suspected. Using my 300mm 4.5 has a shallower DoF than a 200mm with 2.8. So now I know that part of my focus problem is a DoF problem. I need to adjust the focal length and / or the f-Stop to get a bit deeper DoF. Thanks!

    GT
     
  7. DavidSR

    DavidSR TPF Noob!

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  8. ksmattfish

    ksmattfish Now 100% DC - not as cool as I once was, but still

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    Increasing focusing distance also increases DOF.
     
  9. Dao

    Dao No longer a newbie, moving up!

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    Also, there are some JAVA based Depth of Field calculator applications for mobile phone (free). So you can use it anywhere.
     
  10. chrisburke

    chrisburke TPF Noob!

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    ahhh, it looks like calculus class... *runs in the corner, curls up in a ball and screams "theres no place like home, theres no place like home"*
     

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