Page 1 of 2 12 LastLast
Results 1 to 10 of 11

Thread: Calculating f-stop progression . . .how are the numbers derived?

  1. #1
    Drew Bedo's Avatar
    Join Date
    Oct 2006
    Location
    Houston Texas
    Posts
    3,225

    Calculating f-stop progression . . .how are the numbers derived?

    I have noodled around online, but can't quite find out what I would like to know:

    How are the f-stop numbers derived? i\I understand that it is a ratio of focal length to aperture . . .but can't quite get the math right. I have inklings of the natural log of 2 being involved somehow.



    I do not have any pressing reason to do this, but its still early and the coffee has kicked in.
    Drew Bedo
    www.quietlightphoto.com
    http://www.artsyhome.com/author/drew-bedo




    There are only three types of mounting flanges; too big, too small and wrong thread!

  2. #2

    Join Date
    Sep 2008
    Location
    Sarnia, Ontario, Canada
    Posts
    826

    Re: Calculating f-stop progression . . .how are the numbers derived?

    Drew,

    I believe it is a progression of the square root of 2.

    But would be interested in hearing what the right answer might be as well.

    Len

  3. #3

    Join Date
    Dec 2001
    Location
    NJ
    Posts
    8,470

    Re: Calculating f-stop progression . . .how are the numbers derived?

    Quote Originally Posted by Len Middleton View Post
    Drew,

    I believe it is a progression of the square root of 2.

    But would be interested in hearing what the right answer might be as well.

    Len
    That's right. Each full stop step halves (going up from 1.0 to 1.4, 1.4 to 2.0, ...) the amount of light the film receives given subject illumination and shutter speed.

  4. #4

    Join Date
    Nov 2017
    Posts
    654

    Re: Calculating f-stop progression . . .how are the numbers derived?

    Aperture controles light intensity. So it controls the SURFACE of the opening of the diaphragm. But it is expressed as a ratio of linear dimensions. Doubling the surface is done by making the diameter square root of 2 larger. So you get a progression by square root of 2: 1-1.4-2-2.8-4...
    Expert in non-working solutions.

  5. #5

    Join Date
    Jun 2004
    Posts
    59

    Re: Calculating f-stop progression . . .how are the numbers derived?

    A pretty much complete explanation:
    https://www.slrlounge.com/a-thorough...ehind-f-stops/
    For me the key was remembering that doubling the area of a lens opening, and thereby the amount of light, is accomplished by changing the diameter by the square root of 2 (1.4 rounded)

  6. #6
    Nodda Duma's Avatar
    Join Date
    Nov 2014
    Location
    Batesville, Arkansas
    Posts
    1,116

    Re: Calculating f-stop progression . . .how are the numbers derived?

    As Havoc notes, f/# values change by the area size. As such, the numbers change by the square root if 2.
    Newly made large format dry plates available! Look:
    https://www.pictoriographica.com

  7. #7
    Lascassas, TN
    Join Date
    Jan 2006
    Location
    Lascassas TN
    Posts
    262

    Re: Calculating f-stop progression . . .how are the numbers derived?

    On a side note, the longer the lens focal length, the larger the glass surface area must be to give the same f/stop. The diameter of the aperture on a 360mm lens is considerably larger than on a 50 mm lens.
    Bill Kumpf

  8. #8

    Join Date
    Jan 2002
    Location
    Besançon, France
    Posts
    1,617

    Re: Calculating f-stop progression . . .how are the numbers derived?

    Hello from France!
    The present f-number system was not standardized when manufacturers started to sell photographic optics.

    See this comparative chart of old f-stop systems

    And after the f-number was adopted, it took again some time before all lenses would be engraved (except for max aperture) according the modern standard series 1.4 - 2 - 2.8 - 4 - 5.6 - 8 - 11 - 16 - 22 - 32 - 45 - 64

  9. #9

    Join Date
    Nov 2008
    Location
    Atlanta, GA
    Posts
    669

    Re: Calculating f-stop progression . . .how are the numbers derived?

    Another way of looking at it is the progression of the area of a sphere with change in radius, where radius is the f number in any units (inches, meters, furlongs). The area doubles with each f number. The area of a sphere is given by 4 pi r^2, so taking the ratio of 2 radii r1 and r2 yields r1^2/r2^2.

  10. #10

    Join Date
    Dec 2001
    Location
    NJ
    Posts
    8,470

    Re: Calculating f-stop progression . . .how are the numbers derived?

    Quote Originally Posted by Jim Michael View Post
    Another way of looking at it is the progression of the area of a sphere with change in radius, where radius is the f number in any units (inches, meters, furlongs). The area doubles with each f number. The area of a sphere is given by 4 pi r^2, so taking the ratio of 2 radii r1 and r2 yields r1^2/r2^2.
    surely the area of a circle is all that's needed

Similar Threads

  1. Calculating f stop
    By mylek in forum Lenses & Lens Accessories
    Replies: 13
    Last Post: 30-Jan-2016, 18:53
  2. Calculating f stop of Telephoto lens
    By IanG in forum Lenses & Lens Accessories
    Replies: 31
    Last Post: 28-Nov-2014, 13:28
  3. Replies: 5
    Last Post: 25-Apr-2013, 06:37
  4. Calculating f-stop for Achromatic Meniscus Lens
    By Jon Wilson in forum Lenses & Lens Accessories
    Replies: 2
    Last Post: 23-Feb-2011, 07:47
  5. Calculating Optimal F-stop for 8x10
    By G Benaim in forum Cameras & Camera Accessories
    Replies: 2
    Last Post: 18-Oct-2007, 07:37

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •