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Thread: Why is aperture backwards??

  1. #1

    Question Why is aperture backwards??

    So, I get that larger f numbers like f/22 actually mean that it's a smaller opening and that smaller f number like f/2 is a big opening. I learned that thanks to a really helpful beginner video I just watched - this video. But my question is this...WHY is the size of the aperture backwards from the numbers. Shouldn't f/22 be a big aperture since it's a big number and f/2 should be a small aperture since it's a small number???

  2. #2
    Vaughn's Avatar
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    Re: Why is aperture backwards??

    The f/stop value is derived from an equation: It is the focal length divided by the diameter of entrance pupil. Pretty simple, but it can get complicated with some lenses and where to exactly measure the entrance pupil which is not always the same as the aperture.

    It allows comparisons across focal lengths and aperture sizes without have to reference both every time one meters. Makes them easier to use. It is just the way the maths work out.
    "Landscapes exist in the material world yet soar in the realms of the spirit..." Tsung Ping, 5th Century China

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    Tin Can's Avatar
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    Re: Why is aperture backwards??

    Fractions
    2022

  4. #4
    Drew Bedo's Avatar
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    Re: Why is aperture backwards??

    Think of it like fractions: 1/3 of a Pizza is a LARGER portion than 1/8 of a Pizza.

    If that does not help you understand the inverse relationship, some one else here will probably explain the math . . . .by chapter and verse . . .with reference to the natural logarithm of 2 (ln2). . . . .or perhaps it is the square root of 2.
    Last edited by Drew Bedo; 26-Feb-2021 at 11:09.
    Drew Bedo
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    Drew Bedo's Avatar
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    Re: Why is aperture backwards??

    Now: Will someone describe how to determine the exact dat of mardi Grass for any year by making specific reference to the phases of the moon and the Vernal Equinox?
    Drew Bedo
    www.quietlightphoto.com
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    There are only three types of mounting flanges; too big, too small and wrong thread!

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    Alan Klein's Avatar
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    Re: Why is aperture backwards??


  7. #7

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    Re: Why is aperture backwards??

    Well, it was erutrepa for the longest time ...... then Fuji cancelled it.

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    Drew Bedo's Avatar
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    Re: Why is aperture backwards??

    I apologize for the condescending response to a valid question form a new commer.

    CarterFrost: Check your PMs please.
    Drew Bedo
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    There are only three types of mounting flanges; too big, too small and wrong thread!

  9. #9
    Tracy Storer's Avatar
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    Re: Why is aperture backwards??

    As others have said: the "f/#" is a ratio of opening size to focal length. (simplistically, usingd in the same units of measure) A smaller opening "goes into" the focal length more times than a large opening. I hope that is clear enough.

    Quote Originally Posted by CarterFrost245 View Post
    So, I get that larger f numbers like f/22 actually mean that it's a smaller opening and that smaller f number like f/2 is a big opening. I learned that thanks to a really helpful beginner video I just watched - this video. But my question is this...WHY is the size of the aperture backwards from the numbers. Shouldn't f/22 be a big aperture since it's a big number and f/2 should be a small aperture since it's a small number???
    Tracy Storer
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    www.mammothcamera.com

  10. #10

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    Re: Why is aperture backwards??

    Just to add a point of interest for the OP maybe:

    The "f" in the f/8, etc. is the focal length of whatever lens your using. Replace that with the actual focal length of a particular lens and you get a division problem, e.g., 150mm/8 = 18.75, which is the actual size of the entrance pupil at that aperture setting. As you can see, 150mm/4 is larger: 37.5, and 150mm/16 is going to be smaller: 9.375.

    Also, f/8 on two different focal-length lenses will come out differently. 150mm/8 = 18.75, 300mm/8 = 37.5. You can see that the longer focal length lens has a larger-size aperture at the same f-stop. This larger aperture will let in more light, but the fact that the longer lens has to be farther from the film to focus dims that extra light before it reaches the film and exactly compensates for it so that f/8 = f/8 as far as exposure goes for lenses of different focal lengths.

    Best,

    Doremus

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