What is the difference between "Aperture" and "F-Stop"

Discussion in 'Digital Photography' started by Gary Edstrom, May 9, 2005.

  1. Gary Edstrom

    Gary Edstrom Guest

    In looking at the EXIF headers of my pictures, I notice separate entries
    for "Aperture" and F-Stop". Sometimes these entries are the same, and
    sometimes they are different. I have tried looking it up in several
    glossaries, but haven't found anything that really explains the
    difference.

    Thanks, Gary
     
    Gary Edstrom, May 9, 2005
    #1
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  2. Gary Edstrom

    Jim Townsend Guest

    F stop and aperture are essentially the same thing.. The aperture is
    measured in stops. The more stops, the smaller the opening.

    Many cameras have duplicate fields in the EXIF info.. For example, my
    Canon 10D has both "Exposure Time" and "Shutter Speed".

    I don't know why they do this :) I'm guessing it's done to maintain
    backwards compatibility with earlier EXIF standards.
     
    Jim Townsend, May 9, 2005
    #2
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  3. Gary Edstrom

    Paul Furman Guest


    Aperture is the diameter of the opening, f/stop is aperture divided by
    focal length.

    But I think what you are looking at is "maximum aperture of the lens"
    and "f-stop used for this photo".
     
    Paul Furman, May 9, 2005
    #3
  4. Gary Edstrom

    Backbone Guest

    Aperture and F-Stop are fundamentally the same i.e. the higher the number the
    smaller the lenz diaphragm inside the lens.

    I believe what your seeing is the actual lenz aperture and the aperture setting
    depicted at the cameras end. your camera may have a different reading than your
    lenz may represent. lenz aperture will be the true aperture!
     
    Backbone, May 9, 2005
    #4
  5. Gary Edstrom

    peter Guest

    Another way to explain is:
    aperture vs. f-stop is like
    length vs. inches
     
    peter, May 9, 2005
    #5
  6. I'm sure it varies a lot by camera. With my Fuji S2, I get:

    ExposureTime: 1/125
    ShutterSpeed: 1/125
    ShutterSpeedValue: 1/128

    Aperture: 13.0
    ApertureValue: 13.5
    FNumber: 13.0

    (actual values from my photo at
    <http://www.dd-b.net/perl/picpage/dd...ally?id=ddb 20050507 010-029;IPTC=no;EXIF=raw>).

    I'm interested in the pattern of two the same, one slightly different,
    with the different one having the word "value" in the name. But I
    don't know what, if anything, it means.
     
    David Dyer-Bennet, May 9, 2005
    #6
  7. Gary Edstrom

    Ron Hunter Guest

    Moreover, they aren't always exactly the same, at least on my camera.
     
    Ron Hunter, May 9, 2005
    #7
  8. Gary Edstrom

    Jim Townsend Guest

    Yes, I've noticed that too.. But they are usually very close.

    It seems at least one value is the 'raw' value (with decimal places) and
    the other is nicely rounded.
     
    Jim Townsend, May 9, 2005
    #8
  9. Gary Edstrom

    Gary Edstrom Guest

    Ok, I think I have found the answer to my own question:

    And it involves MATH!

    Take the square root of 2.

    Raise it to the power of the aperature.

    The result is the F-Stop.

    I have tried this on several of my pictures and it works very precicely.

    Example: Aperture=6.9189 F-Stop=11

    (Sqrt(2))^6.9189 = 11.00014

    I have tried it on other pictures and it seems to work.
     
    Gary Edstrom, May 9, 2005
    #9
  10. Gary Edstrom

    MitchAlsup Guest

    The problem is that there ate 2 kinds of aperture!

    Linear aperture is the size of the opeinging that the light passes
    throught.
    Relative aperture is F/stop which is focal length divided by linear
    aperture.

    Astronomers use aperture (casually) to mean linear aperture
    Photographers use aperture (causually) to mean F/stop
     
    MitchAlsup, May 9, 2005
    #10
  11. Gary Edstrom

    Jack M Guest

    Always wondered the same thing...Thanks for the info
     
    Jack M, May 9, 2005
    #11
  12. Gary Edstrom

    Neil Ellwood Guest

    That is a little more complicated than what I had always understood.

    That the f no. = the number of tomes that the diameter of the aperture
    will divide into the focal length of the lens.
     
    Neil Ellwood, May 9, 2005
    #12
  13. Gary Edstrom

    Gary Edstrom Guest

    Here is a conversion table to convert from Aperture to F-Stop. This
    seems to match the EXIF data generated by my Canon EOS 20D. Fractional
    apertures would of course fall somewhere between 2 F-Stops. The term
    "Aperture" as used in the EXIF header seems to be a much different
    aperture than we would normally think of. It is a logarithmic scale.
    Each increase of 1 unit on the aperture scale results in a cutting in
    half of the light to the CCD.

    Gary

    Aperture F-Stop

    0 1
    1 1.4
    2 2
    3 2.8
    4 4
    5 5.6
    6 8
    7 11
    8 16
    9 22
    10 32
    11 45
    12 64
     
    Gary Edstrom, May 9, 2005
    #13
  14. Gary Edstrom

    Nostrobino Guest

    Other way around. Focal length divided by size of aperture.

    For example, f/8 says the aperture size is f (the focal length) divided by
    8.

    N.
     
    Nostrobino, May 12, 2005
    #14
  15. Gary Edstrom

    Nostrobino Guest

    I'm not sure what you're doing there, but the standard f-stop series is
    based on multiples of the square root of 2, such that each higher number
    represents half the *area* of the aperture opening and thus passes half as
    much light in the same period of time. (If it were half the *diameter* it
    would have only one-fourth the area and thus pass one-fourth the amount of
    light.)

    Starting with 1, the series is 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16 and so on.
    Thus f/2.8 passes half as much light as f/2 but twice as much as f/4, four
    times as much as f/8 and so on. There are in-between stops of course.

    N.
     
    Nostrobino, May 12, 2005
    #15
  16. Gary Edstrom

    Paul Furman Guest


    Ah, thanks, that explains the fractional expression of f-stop. Isn't
    that nice when things make sense.
     
    Paul Furman, May 13, 2005
    #16
  17. Gary Edstrom

    JPS Guest

    In message <>,
    Not exactly. When aperture is referred to by an f-stop, you have to
    know the focal length to know what the aperture actually is. Aperture
    is measured in millimeters' f-stop is a ratio of focal length to
    aperture.
    --
     
    JPS, May 13, 2005
    #17
  18. Gary Edstrom

    Nostrobino Guest

    Oops. That should have read, ". . . four times as much as f/5.6, eight times
    as much as f/8 and so on."

    N.

    There are in-between stops of course.
     
    Nostrobino, May 13, 2005
    #18
  19. Gary Edstrom

    Nostrobino Guest

    You're welcome, and yes it is. ;-)

    N.
     
    Nostrobino, May 13, 2005
    #19
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