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Thread: Need f-stops explained

  1. #1

    Need f-stops explained

    Please let me know if my understanding of f-stops is correct. Is it correct to say that the f-stop number has no value? It is simply a number. To say that an f-stop setting of f/11 is a certain diameter aperture opening, is not correct. The size of the aperture opening is dependent upon the focal length of the lenses being used. For example, if I am using a 100mm lens, and I have the aperture set at f/11, then the diameter of the aperture opening is 100/11 = 9.1mm. And if I have a lens with a focal length of 200mm and I am still set at f/11, then the resulting aperture opening would be 200/11 = 18.2mm.

    If this is not correct, please straighten me out.

    Thanks,

    Tom Iskiyan

  2. #2
    Zone XI's Avatar
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    Re: Need f-stops explained

    You are correct

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    Re: Need f-stops explained

    Need f-stops explained

    I'm not very good writing with a mouse after a barrel of geer. Yes I think you are right.

    The numbers are just powers of square root of 2, and correspond to halving/doubling area and hence doubling/halving of exposure.
    Last edited by arith; 9th November 2011 at 10:17 AM.

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    Re: Need f-stops explained

    F-stops are a ratio of the hole in the lens to the focal length. As such they have no units. But they are useful.

    You can use the f-stop to compare how much light can get through different lenses. Since they are a ratio they can be compared between lenses of different focal lengths. E.g. a 100mm f2.8 and 200mm f2.8 will let through the same amount of light onto the sensor (just the 200mm will have a hole that is twice as wide). Beware though since such comparisons can soon lead to lens envy.

    The relationship between f-stop and light is a fundamental part of setting the exposure correctly on your camera. If you know the exposure is correct at a given ISO, f-stop and shutter speed then you can work out what settings to change to get the correct exposure at a different ISO or f-stop or shutter speed.

    Alex

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    Re: Need f-stops explained

    Quote Originally Posted by herbert View Post
    (just the 200mm will have a hole that is twice as wide)Alex
    If I'm not mistaken it's the area that will be twice as large - not the width.

    Sorry, couldn't help myself.

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    Re: Need f-stops explained

    The science and maths of it all just baffle me. I just know what sort of depth-of-field I'll get with the different aperture values. That's all I need to know. Any more would just confuse me!

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    Re: Need f-stops explained

    The fstop number is actually the fraction that the diameter of the aperture hole is of the focal length of the lens, So f2 on say a 100mm lens is physically 50mm in diameter, in other words, one half of the focal length. Which is why the aperture on a zoom lens has a different number according to the focal length the lens is set at at the time. The hole itself is exactly the same size all the time, it's only its fraction of the focal length that changes

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    Re: Need f-stops explained

    Quote Originally Posted by The Stig View Post
    If I'm not mistaken it's the area that will be twice as large - not the width.

    Sorry, couldn't help myself.
    Kind of:

    N = f / D

    N is the f-number (f/#)
    f is the focal length
    D is the diameter of the entrance pupil

    So if the lens is twice as long, the entrance pupil is twice as wide for the same f-number.

    You are right though that the area doubles when you increase the lens by a stop. As Steve's scribbles show area is related to the square of the radius. So to double the area you have to increase the radius by sqrt(2), or 1.4.

    When stopping down a lens this leads to the familiar 1, 1.4, 2, 2.8, 4, 5.6, 8, etc. sequence. The effective lens entrance pupil is reduced in diameter by 1.4, but the total area is reduced by half. The light falling on the sensor is thus reduced by half.

    Not that it really matters to your pictures. I think it is more important to understand the qualities of the output image for each aperture setting on your lens. How does it shoot wide open, where is it sharpest, how much do you need to close it down to cover all the objects of interest with good focus.

    Alex

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    Re: Need f-stops explained

    F-Stops Explained ... all we need to know.

    - The lower the number - the bigger the hole - the more light gets let in - Depth of Field decreases

    - The higher the number - the smaller the hole - the less light gets in - Depth of Field increases

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    Re: Need f-stops explained

    Well said Colin.

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    Re: Need f-stops explained

    Quote Originally Posted by Colin Southern View Post
    F-Stops Explained ... all we need to know.

    - The lower the number - the bigger the hole - the more light gets let in - Depth of Field decreases

    - The higher the number - the smaller the hole - the less light gets in - Depth of Field increases
    AND... All you have to remember is f/1.4 and f/2 and you can always know the spectrum of full f/stops...

    Just double each successive number for the next full f/stop which will cut the light transmitted in half and require 2x the exposure time...

    f/1.4 and f/2 Now start doubling these and you get:

    f/2.8 (which is remembered by doubling f/1.4)
    and requires 2x more exposure time as f/2

    f/4 (which is remembered by doubling f/2)
    and requires 2x more exposure time as f/2.8

    f/5.6 (which is remembered by doubling f/2.8)
    and requires 2x more exposure time as f/4

    f/8 (which is remembered by doubling f/4)
    and requires 2x more exposure time as 5.6

    f/11 (which is remembered by doubling f/5.6)
    and requires 2x more exposure time as f/8

    f/16 (which is remembered by doubling f/8)
    and requires 2x more exposure time as f/11

    f/22 (which is remembered by doubling f/11)
    and requires 2x more exposure time as f/16

    An aperture of f/22 is just about as small as the apertures go for most DSLR lenses but older and longer focal length lenses (especially for large format film cameras) could have f/32, f/45, f/64 and even f/128 which is a continuation of the doubling system...
    Last edited by rpcrowe; 10th November 2011 at 11:45 PM.

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    Re: Need f-stops explained

    I think the only use of knowing this has apart from being able to work out exposure; is if some kid asks what's the strange numbers, ya don't have to be cos ya don't know.

    Can you imagine flying an airplane and not knowing what the strange hieroglyphics on the dashboard mean, even if you use autopilot.

    So it doesn't hurt to know this easy stuff, I mean there is a lot harder stuff.

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    rpcrowe's Avatar
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    Re: Need f-stops explained

    Quote Originally Posted by arith View Post
    I think the only use of knowing this has apart from being able to work out exposure;
    Steve, you are so right but, being able to figure out exposure is the basis of photography. If a photographer has a general idea of exposure, he or she can figure out if the exposure being used is somewhere close. We sometimes see posts from photographers who are using ten times or more the exposure needed for a scene without even realizing that they are doing that and they wonder why their image is completely white.

    Sure we all have accurate exposure meters in our cameras but that doesn't mean that we shouldn't be able to pretty well determine exposure on our own. After all, owning a calculator doesn't make knowing how to add and subtract, multiply and devide obsolete.

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    Re: Need f-stops explained

    Quote Originally Posted by herbert View Post
    N = f / D
    Egg, meet face (on my part)

    Apologies, Alex

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    Re: Need f-stops explained

    Quote Originally Posted by The Stig View Post
    Egg, meet face (on my part)

    Apologies, Alex
    No problem. Everyone makes mistakes (I certainly do). We are all here to learn in a friendly atmosphere for sharing our passion for photography.

    I just hope this thread has helped some people understand f-numbers a bit more. It was all a mystery to me when I bought my first camera. I know enough now to improve my photography in general (and my purchases for next time).

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    Re: Need f-stops explained

    Quote Originally Posted by Colin Southern View Post
    F-Stops Explained ... all we need to know.

    - The lower the number - the bigger the hole - the more light gets let in - Depth of Field decreases

    - The higher the number - the smaller the hole - the less light gets in - Depth of Field increases
    That's all I need or want to know. Math has never been my thing....

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    Re: Need f-stops explained

    Quote Originally Posted by Colin Southern View Post
    F-Stops Explained ... all we need to know.

    - The lower the number - the bigger the hole - the more light gets let in - Depth of Field decreases

    - The higher the number - the smaller the hole - the less light gets in - Depth of Field increases
    Quote Originally Posted by Kris V View Post
    That's all I need or want to know. Math has never been my thing....
    My thoughts exactly... Need f-stops explained

  18. #18
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    Re: Need f-stops explained

    Tom there are some great explanations here. Could I also just mention that with a lot of modern cameras, there are also intermediate values of f stops available. ie between the traditional full stop values. eg between f/11 and f/16 you can get f/13 and f/14.

    Cheers Dave

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    Re: Need f-stops explained

    Quote Originally Posted by arith View Post
    Can you imagine flying an airplane and not knowing what the strange hieroglyphics on the dashboard mean, even if you use autopilot.

    So it doesn't hurt to know this easy stuff, I mean there is a lot harder stuff.
    On the other hand, do you need to know every nut and bolt on a car to be able to drive it?

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    Re: Need f-stops explained

    Quote Originally Posted by arith View Post
    Can you imagine flying an airplane and not knowing what the strange hieroglyphics on the dashboard mean, even if you use autopilot.
    It's been said that the aeroplane of the future will have only a single pilot and a dog in the cockpit. The pilot's sole job is to feed the dog. The dog's sole job is to bite the pilot if he touches anything!

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