F/stop vs. F/stop issue quizz?

Discussion in 'General Technical Discussion' started by Greg, Apr 7, 2005.

  1. Greg


    Apr 5, 2005
    Fayetteville, TN
    OK, let's see how sharp this group is. This should be easy.

    1. Given two lenses with the same focal length, will an F/6.4 have the same depth of field as an F/2.8 when the F/2.8 is "stopped down to 6.4?

    2. What part does the diameter of the lens play?
  2. Flew


    Jan 25, 2005

    I a noob, so I'll probably get these totally wrong, but I'm going to play anyway. :lol: What better way to learn than to flunk a test or two. 8)

    1. They should be the same.

    2. None.


  3. dkapp


    Mar 18, 2005
    San Francisco
    Here's my guess.

    1.) They are the same.

    2.) It will allow faster and more accurate focusing in low light conditions.

  4. Flew


    Jan 25, 2005
    I agree with Dave on #2. My answer was based strickly on how the lens diameter affects DOF. 8)

  5. I think that is the perfect attitude and so in the same spirit I'm going to hazard two guesses...

    1 - I'm guessing this is to do with efficiency and tolerances with one lens at its limit and the other with capacity to spare. So I think the DOF will be the same but the stopped down lens will render the entire image sharper and will therefore appear to have the greater DOF.

    2- I think this is something to do with the efficiency of light "collection" and so would go with a guess that the larger the diameter the better the lens.
  6. nfoto

    nfoto Guest

    1. DOF is identical (I presume both lenses are focused at the same subject at the identical distance)

    2. None. The f/number (N) is defined as the diameter of the entrance pupil divided into the focal length. The entrance pupil may exceed the physical lens diameter.
  7. This DOF calculator answers both question.


    The only input data it requires is distance to subject, focal length of lens, and aperture used.

    I like Clive's comment, too. I'd also speculate that two lenses at the same aperture/focal length might yield results that suggest different DOFs because of the varying natures of their bokeh.
  8. Greg


    Apr 5, 2005
    Fayetteville, TN

    good job guys but it was an easy one.

    Hey Frank, Thanks for the link. May come in handy some day.
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