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Stupid f/stop question

Discussion in 'General Discussion' started by Maestrodawg1, Oct 1, 2005.

  1. Here is an unimportant question that I have always wondered but never really asked. Why is apeture/f/stop measured the way it is?

    I mean the fastest lense I have is a f/2.8. I click it back once and get 3.2. then 3.5 then 4 then 4.5 etc.

    Starting at f/8, I then get whole numbers. Really the measuement of an f/stop boggles my mind. Why the decimals? It really isn't that important because I understand the practical aspects of f/stop settings, just not why it is numbered so.
  2. nfoto

    nfoto Guest

    It's a series involving an irrational number (1.414 ... = square root of 2), hence the decimals. Each f-number doubles (or halves) the light coming through the lens aperture, and since there is an "area" term involved, you get that nasty sqrt(2) factored into the scale.

    Traditionally the steps are rounded to give "nice" numbers and some steps are so nice they really are way off their true value as well :biggrin:

    In case you wondered about it, the f-scale goes like this (whole steps);

    0.5, 0.7, 1.0, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45, 64, 90, 128, 180, 256, ....

    The inaccurate numbers we encounter in practice are as follows,

    5.6 (should be rounded to 5.7),
    11 (should be 11.3)
    22 (should be 22.6)*
  3. That's awesome Bjorn! Thanks for sharing that info. I've occassionally wondered about the numbering myself.
  4. Iliah


    Jan 29, 2005
    Aperture value is the ratio between entrance diameter of the lens and focal length. Lens of 50mm focal length should have front lens diameter of 50mm to have maximum opening of f=1; or 18mm lens diameter to have maximum opening of f=2.8; 36mm front lens for maximum aperure of 1.4.

    But light actually enters through surface area. We have that (S =Pi x r x r) formula. If you change the aperture value (in fact, radius) by sqrt(2), surface area and amount of light accordingly will change by 2 times.
  5. Iliah


    Jan 29, 2005
    Sorry, Bjørn - you answered while I was typing :) 
  6. nfoto

    nfoto Guest

    It's not the front element diameter that is involved, it's the size of the entrance pupil. So what is this "pupil"? If you were a school master you give one of your pupils a lens and ask him to hold it at arm's length and look into it. What the pupil sees is the entrance pupil, and were the pupil a bright pupil, he'd turn the lens around and look into the lens again, just to see the exit pupil.

    The significance of these pupils (the optical ones) is that the entrance pupil governs the light collecting ability of the lens and the exit pupil illuminates the medium on which the image is recorded. The ratio between them is called pupil factor and is significant in changing the aperture of the lens from its nominal value (valid at infinity) to the "effective" aperture which accounts for light loss when the lens is focused close.

    Now, time for a recess. :cool: 
  7. Iliah


    Jan 29, 2005
    Yes, Bjørn :)  If the lens is composed of several lenses, I think :) 
  8. nfoto

    nfoto Guest

    If the lens is just a single element, then the entrance and exit pupils are identical in size. Many* of our lenses don't fulfil that requirement, I think.

    Pupil factor p, defined as the ratio exit pupil/entrance pupil, is instrumental in controlling close-up exposure. Thus, a telephoto lens (with p < 1) or a wide-angle lens (with p > 1) can give several stops of different exposure for the same image magnification. Many people are unaware of this basic fact.

    (* edited: my English was written faster than my mind ought to allow)
    Last edited by a moderator: Oct 1, 2005
  9. Gale


    Jan 26, 2005
    Viera Fl
    Bjorn and Iliah,
    I have two pupils which I have to use glass in front of, to see all the pupils in the class above...hummm or was that people :>))

    Yes, time for recess :>)))))

    thanks you guys.
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