What Does The "f" in f/x.x stand For?

Discussion in 'Photography Equipment & Products' started by blakers81, Mar 30, 2010.

  1. blakers81

    blakers81 TPF Noob!

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    In another thread, I read a post that said one of the key things to look for when taking good pictures of moving objects is a low f/x.x

    I am just wondering what this stands for? And, I thought when you dish out the extra money and buy a better/stronger lens, it would have a lower f/x.x, but from checking some zoom lens, this seems not to be the case.

    For instance, the Nikon D5000 I am looking at says 18-55 mm f/3.5-5.6 VG, and then another lens for it says 55-200 mm f/4-5.6 VG.

    I remember reading the post about this and the poster saying that the key is to buy a camera with an f/x.x like in the 2 or 1 range.

    Any thoughts on this?

    Thanks.
     
  2. Formatted

    Formatted TPF Noob!

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    Smaller the F-stop the larger the aperture. Which means more light coming. So a larger F-stop the more light that can get in!
     
  3. blakers81

    blakers81 TPF Noob!

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    Did you mean to write the SMALLER the F-stop, the more light that can come in? It sounds like you contradicted yourself in the two sentences you wrote.
     
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    Formatted TPF Noob!

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    Yes I did, the smaller the F-Stop the larger opening so the more light that comes in.

    It is completely confusing!
     
  5. blakers81

    blakers81 TPF Noob!

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    Ok thanks.
     
  6. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    F-number - Wikipedia, the free encyclopedia

    The F number is a ratio between the entrance pupil of the lens, and the focal length. So when we say F8, it's actually 1:8 or 1/8 etc.

    So F4 is actually 1:4 (1/4), which is physically larger than 1:8.
     
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  7. PhotographyPool

    PhotographyPool TPF Noob!

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    It's a ratio
     
  8. Formatted

    Formatted TPF Noob!

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    Thanks for doing a three word post to plug your website...
     
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  9. KmH

    KmH Helping photographers learn to fish Supporting Member

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    The simple answer is f=1, and 1 = the focal length of the lens.
    It's written that way on lenses too, 1:2.8, to denote the ratio of the lens focal length to the lens aperture that it is.
     
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  10. WayneF

    WayneF No longer a newbie, moving up!

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    The number value called f/stop is defined as the ratio of the lens focal length to the effective aperture diameter.

    f/stop = focal length / aperture diameter.

    A lens with say focal length 50 mm and an effective aperture diameter of 5 mm would be a f/10 lens aperture setting.

    This beauty of this convention is that it allows the same f/stop on any two lenses, say both set at f/8, to give the same exposure, regardless of focal length.
    And furthermore, we learn to know how much f/8 is.

    It is commonly written as the format: f/2.8, to be remindful of the aperture diameter represented by this division of focal length by the f/number. Turns out we really don't sweat the actual diameter, this f/stop ratio number tells us what we need to know about the exposure capability.

    It seems reversed until you get used to it, because of this division.

    It means a large aperture diameter gives a low f/stop number (like say f/2.8, which allows more light into the exposure).

    A small aperture diameter gives a high f/stop number (like say f/16, which allows much less light into the exposure).

    See Photographic Tables, F/stop, Shutter Speed, ISO and EV for more detail about the details. :)
     
  11. 480sparky

    480sparky Chief Free Electron Relocator Supporting Member

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    We can't?
     
  12. D-B-J

    D-B-J Been spending a lot of time on here!

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    I
     

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