What is a F stop

Discussion in 'Photography Beginners' Forum' started by SethAlbritton, Jun 23, 2010.

  1. SethAlbritton

    SethAlbritton TPF Noob!

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    When people say go down a f stop, what does that mean.

    Say I'm at f 2.8 what is an fstop up.

    I may be wrong, but I think you can substitute things. What I mean is cant you go up an f stop on your flash by going dow an f stop somewhere else. Any help on the topic of fstops will help.
     
  2. pmsnel

    pmsnel TPF Noob!

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    Google? Wikipedia?

    Of Wikipedia:
    On modern cameras, especially when aperture is set on the camera body, f-number is often divided more finely than steps of one stop. Steps of one-third stop (1/3 EV) are the most common, since this matches the ISO system of film speeds. Half-stop steps are also seen on some cameras. As an example, the aperture that is one-third stop smaller than f/2.8 is f/3.2, two-thirds smaller is f/3.5, and one whole stop smaller is f/4.
     
  3. pmsnel

    pmsnel TPF Noob!

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    • Like Like x 1
  4. benlonghair

    benlonghair TPF Noob!

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    In general, a 'full stop' refers to the doubling or halving of light entering the camera or in the case of aperture doubling or halving the area of the aperture. For example:

    50mm @ f/5.6 = aperture opening of 250sq mm give or take a little.
    50mm @ f/8 = aperture opening of 122.6sq mm (or about half that of 5.6)
     
  5. ifi

    ifi TPF Noob!

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  6. GregR

    GregR TPF Noob!

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  7. KmH

    KmH Helping photographers learn to fish Supporting Member

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    There can also be a stop of shutter speed, and a stop of ISO.

    Anything exposure value that is doubled or halved is a stop.

    1/100 to 1/200 is a stop of shutter speed.

    ISO 200 to ISO 400 is a stop of ISO.

    In the case of aperture a stop is a function of the square root of 2 (1.41421).

    f/1 times 1.4 = 1.4 (f/1.4)
    f/1.4 times 1.4 = 1.96 (f/2)
    f/2 times 1.4 = 2.8 (f/2.8)
    f/2.8 times 1.4 = 3.92 (f/4)
    f/4 times 1.4 = 5.6 (f/5.6)

    So full stop steps are f/1 > f/1.4 > f/2 > f/2.8 > f/4 > f.5.6, ......

    Each smaller stop (aperture) lets in 1/2 as much light. Each larger stop lets in twice as much light as the smaller stop did.

    Note: f/1.4 is a bigger number than is f/5.6, because they are fractions (ratios).
     
  8. SethAlbritton

    SethAlbritton TPF Noob!

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    I have googled. I have a basic understanding, but not a very in depth one. I understand f2.8 lets in more light, has alot dof, and all that good stuff. I know how to work a camera and all that jazz. I've just always been confused when I want to substitute iso for flash or something like that. I want to be able to keep the same exposure while changing the iso, aperture, and shutter speed.

    Thank you guys, even you clever people who recommended google.

    pmsml how is 2.8 one stop from 4. Kmh says double or halved = stops

    I'm still confused with the aperture.

    Lets make sure I understand Iso and shutter speed stops.
    1/200 is a slop lower than 1/400
    400 iso is a stop lower than 800
     
  9. lostpacket

    lostpacket TPF Noob!

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    F stop is how big your aperture is open. what is confusing to new people is that a smaller f-stop is actually bigger. Common sense would make you thing f/1.4 is smaller than f/5.6, but it is not.

    Like others have said, each f-stop halves or increases the available light.

    As you go down (a larger number) in f-stops (f/8 to f/11) the opening gets smaller. Hence, why they call it "stopping down." Think of it as would you rather have 1/2 (f/2) a pie or 1/8 (f/8)? The 1/2 is bigger.

    You need to also remember that f-stops are inverses of shutter speeds. If you go down in f-stop, you are reducing light, so you need to increase the shutter by 1 as well. This means you get the exact same amount of light.

    Here is a pic that shows the values
    [​IMG]

    Also, depth of field (what is blurry) is affected by f-stop values. But that is another subject.
    [​IMG]
     
  10. Josh66

    Josh66 Been spending a lot of time on here!

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    It's a little different with aperture since you're talking about the area of a circle instead of some linear measurement.

    Each stop is an increase by a power of √2.
    1.4→2.0→2.8→4.0→5.6→8.0→11→16→22→32→45→64 etc... (see the pattern?)

    f/1.4 = √2^1
    f/2 = √2^2
    f/2.8 = √2^3
    f/4 = √2^4
    f/5.6 = √2^5
    f/8 = √2^6
    f/11 = √2^7

    etc.
     
    Last edited: Jun 23, 2010
  11. Dwig

    Dwig TPF Noob!

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    Aperture = hole = the opening in the lens

    Iris = the common mechanism for controlling the size of the aperture

    f/stop = the unit of measure generally used in photography for the size of the aperture. On photographic equipment it is generally written in fraction notation (f/8 = focal length / 8) but technical optical usage is generally in ratio notation (f/8 = 1:8). Ratio notation is generally seen on the lens specifications engraved around the front of the lens (e.g. "Nikkor-AF 50mm 1:1.8" on a 50mm f/1.8 lens) and in technical specs for telescopes and similar devices, where it is referred to as "focal ratio".
     
  12. AlexL

    AlexL TPF Noob!

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    Gotta love the explanation :)
     

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