Please enhance my knowledge of dof scales

Discussion in '35mm Cameras' started by Monica Schulz, Jun 2, 2007.

  1. Hi,

    Trying to understand the theory behind the dof scales of my Nikon
    lenses I wonder what the distance between two f-stop marks stands for.
    I read in Merklingers " The Ins and Outs of FOCUS" that the dof scale
    is a ruler tha measures distances in the unit of the max allowable
    circle of confusion. But the distance between the f-stop marks is not
    the diameter of the circle of confusion, which is 0,03 mm in the case
    of Nikon. If I divide the distance between the focus mark and the mark
    for f/22 on my f/2,8 24 mm its 11 mm and 11 mm/22= 0,5 mm. Maybe the
    distance is coc*the pitch of the helicoid of the lens barrel?! Does
    someone know better?

    Best regards!
    Monica
     
    Monica Schulz, Jun 2, 2007
    #1
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  2. Monica Schulz

    Alan Browne Guest

    Funny.

    Two posts via google groups, asking about Nikon DOF issues, and of a
    similar nature from "different people" at

    83.135.226.213 (Bielefeld)
    83.135.221.172 (Herne) http://tinyurl.com/yqejjj

    Cities less than 150km apart in Germany.

    Surely there is a Crop Circle of Confusion in Nordhein-Westfalen !!

    (Why do I get the feeling I know exactly who the poster is?)
     
    Alan Browne, Jun 2, 2007
    #2
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  3. Yes, you need the distance from the film plane. How much you turn
    the lens is irrelevant and is dependant on the pitch of the
    helicoid.

    For 35mm work the relationship of lens movement to CoC is
    of little to no practical interest - just go by the scale on
    the lens. If you want 1/2 the mfg's CoC use 1/2 of the dof
    span marked on the lens.

    The acceptable CoC depends on the photographers desire for
    sharpness, the enlargement ratio and the viewing distance.

    Note that the CoC calculations assume a perfect ray bundle and this
    is not the case in real life. The rays don't come together at
    a point but form a 'sploge' at the film plane at the point of sharpest
    focus. The size of the splodge before and after the film plane
    is not as assumed in the DOF calculations.

    See the various aberrations here:

    http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/aberrcon.html#c1
     
    Nicholas O. Lindan, Jun 2, 2007
    #3
  4. Well done Alan but there is no mystery: Marc is my brother and as we
    share the same interest in photography we take part in posting to the
    usenet what we can´t solve on our own. Anyway, I wonder if there is a
    list out there that connects those ip numbers to the right cities?!

    Best!
    Monica
     
    Monica Schulz, Jun 3, 2007
    #4
  5. I´m sorry but that´s not clear to me. Do you mean the distance between
    the f-stop marks equals or corresponds to the distance from the film
    plane?

    Monica
     
    Monica Schulz, Jun 3, 2007
    #5
  6. On the off chance that this is straight: The DOF marks indicate the
    distance
    limits for acceptable DOF. Say you focus at 5 meters and you are
    shooting at
    f/22. If one of the f/22 marks is now at 3 meters and the other is at
    9 meters then
    the Depth Of Field or range of "acceptable" focus extends from 3
    meters to 9 meters
    when focused at 5 meters at f/22 with that particular lens.
     
    bob.kirkpatrick, Jun 5, 2007
    #6
  7. Monica Schulz

    Draco Guest

    Bob has the right answer for you. The marks on the lens are to show
    the range of in focus with a certain f/stop. They really have not much
    to do with CoC or distance to the film plain to the lens opening. They
    allow you to "preset" your lens at a distance and with the proper f/
    stop you can shot and run and still get an acceptable image.
    The f/stop is in direct relation to the opening size to the lens
    focal length. So a 50mm f/2 lens will measure 25mm across the opening.
    Okay? Nothing to do with the marks on the barrel.


    There isn't any stupid questions. Just some silly answers.


    Draco


    Getting even isn't good enough.

    Doing better does.
     
    Draco, Jun 5, 2007
    #7
  8. Monica Schulz

    Scott W Guest

    But as Nicholas pointed out what is "in focus" is a bit subjective.
    And this will also depend on the camera that is being used. I find
    with either the 20D or the 350D that I have to use about 1 f/stop
    wider on the scale to get what I consider in focus. So for example if
    I am shooting at f/16 I use the f/11 marks on the lens. If I was
    shooting with a FF camera then I would likely find the f/16 marks to
    work much better.

    And if you are using a film camera then it will also depend on what
    film you are using since for example a ISO 800 print film will never
    be close to as sharp as say a 100 ISO slide film.

    Scott
     
    Scott W, Jun 5, 2007
    #8

  9. Thank you Bob and Draco, it´s perfectly clear to me what the dof
    scales are beeing used for.
    What I´m interested in is how the position of the f-stop marks are
    beeing derived. I know that they are based on the max allowable circle
    of confusion that the manufacturer thinks is appropriate. You can
    calculate that value for any lens using the formula that Norman Koren
    has on his website (http://www.normankoren.com/Tutorials/
    MTF6.html#DOFscale):

    coc = f^2 / (Nmax(s-f)

    For most lenses it´s around 0,03 mm. But if I take a lens and measure
    the distances between the marks I find they are larger than this
    value. That was what made me ask if the pitch of the helicoid is
    involved in any case. - Nicholas said "Yes, you need the distance from
    the film plane." I still wonder if this means "Yes it is".

    Best regards!
    Monica
     
    Monica Schulz, Jun 5, 2007
    #9
  10. As far as I know about the theory diffraction and pixel size might
    play a role in this case.
    At f/16 a lens produces a diffraction disk of 0,0217 mm diameter, at f/
    11 it´s diameter is only 0,0149 mm.
    If the 20D or 350D have pixel spacings much smaller than this the
    large diffraction disk will blur the image noticeably.
    And this is really the case. You can see it in a wonderful interactive
    sample on http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm
    ("VISUAL EXAMPLE: APERTURE VS. PIXEL SIZE", middle of the page).

    Best!
    Monica
     
    Monica Schulz, Jun 5, 2007
    #10
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