Aperture number: What does it mean?

Discussion in 'Beyond the Basics' started by prodigy2k7, Jul 20, 2008.

  1. prodigy2k7

    prodigy2k7 No longer a newbie, moving up!

    Joined:
    Apr 22, 2008
    Messages:
    1,665
    Likes Received:
    22
    Location:
    California, USA
    Can others edit my Photos:
    Photos OK to edit
    Wikipedia says:

    "More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane."

    So, im assuming there is some complex calculation or something that has to do with the "cone angle", whatever that is....

    Does anyone have any information on what exactly the numbers mean? Where did they come from etc. Is it from some mathematical formula?

    Any information at all! thanks :)
     
  2. reg

    reg TPF Noob!

    Joined:
    May 28, 2008
    Messages:
    1,487
    Likes Received:
    1
    Location:
    Stuck inside of Mobile with the GTFO Blues Again
    Can others edit my Photos:
    Photos OK to edit
    I'm sure someone else will come in here with WHAT it's compared to because I forget, but it's a measure of a fraction of the opening of the lens.

    f/2 = one half of the full possible opening.
    etc.

    I wanna say that it's based on the same diameter as the filter threads but I don't remember offhand.
     
  3. icassell

    icassell TPF Noob!

    Joined:
    Jun 7, 2008
    Messages:
    9,893
    Likes Received:
    10
    Location:
    Arizona
    Can others edit my Photos:
    Photos NOT OK to edit
  4. reg

    reg TPF Noob!

    Joined:
    May 28, 2008
    Messages:
    1,487
    Likes Received:
    1
    Location:
    Stuck inside of Mobile with the GTFO Blues Again
    Can others edit my Photos:
    Photos OK to edit
    Yeah, he clearly references the wikipedia article in the OP so that doesn't help at all.

    :roll:
     
  5. John_Olexa

    John_Olexa No longer a newbie, moving up!

    Joined:
    Feb 3, 2008
    Messages:
    433
    Likes Received:
    47
    Location:
    Maryland
    Can others edit my Photos:
    Photos NOT OK to edit
  6. christopher walrath

    christopher walrath No longer a newbie, moving up!

    Joined:
    Mar 19, 2008
    Messages:
    1,265
    Likes Received:
    23
    Location:
    In a darkroom far, far away...
    Can others edit my Photos:
    Photos NOT OK to edit
    The aperture number on your lens is the denominator of a fraction that provides the aperture's diameter (or whatever Helen B wants to call it). f/2 = d (where f equals lens' focal length and d equals aperture diameter). So you have a 50mm lens. f/2 (50/2) = a 25mm diameter aperture. f/4 = a 12.5mm aperture diameter. f/8 = a 6.2mm aperture diameter.
     
  7. reg

    reg TPF Noob!

    Joined:
    May 28, 2008
    Messages:
    1,487
    Likes Received:
    1
    Location:
    Stuck inside of Mobile with the GTFO Blues Again
    Can others edit my Photos:
    Photos OK to edit
    Best.
    Answer.
    Ever.
     
  8. epatsellis

    epatsellis TPF Noob!

    Joined:
    Apr 15, 2008
    Messages:
    524
    Likes Received:
    18
    Can others edit my Photos:
    Photos NOT OK to edit
    chris, repeat 50 times, entrance pupil, entrance pupil

    The f.l / entrance pupil (or effective aperture, with apparent mag. by the front element.) is the f. number.

    erie
     
  9. brianne5499

    brianne5499 TPF Noob!

    Joined:
    Feb 18, 2008
    Messages:
    506
    Likes Received:
    1
    Location:
    Crossville, Tennessee
    Can others edit my Photos:
    Photos OK to edit
    Simply Put

    the bigger the number, the smaller the opening...so the less light the that can enter = a longer exposure time

    and of course, it's exactly the opposite at the other end.
     
  10. prodigy2k7

    prodigy2k7 No longer a newbie, moving up!

    Joined:
    Apr 22, 2008
    Messages:
    1,665
    Likes Received:
    22
    Location:
    California, USA
    Can others edit my Photos:
    Photos OK to edit
    I wasn't going for simply put :p
    Thanks! I finally get it now :p

    "
    It's all relative

    A 400mm lens set at f/4 has a pupil diameter of 100mm.
    A 100mm lens, also set at f/4, has a pupil diameter of 25mm.
    "
    http://www.kevinwilley.com/l3_topic01.htm

    So, when talking about a zoom, such as EF 70-200mm F/4 USM, there is a zoom but constant f/4 does that mean the f/4 stats the same but as you zoom the actual physical size changes such at 70/4 = 17.5mm and 200/4 = 50mm. That sounds weird to me...
     
  11. Helen B

    Helen B TPF Noob!

    Joined:
    Sep 16, 2007
    Messages:
    3,296
    Likes Received:
    465
    Location:
    Hell's Kitchen, New York
    Can others edit my Photos:
    Photos NOT OK to edit
    This is one of the reasons that it is important to understand the concept of the entrance pupil. As already mentioned by erie, the f-number is equal to the focal length divided by the diameter of the entrance pupil. This may be written

    N = f/d

    The entrance pupil is the image of the actual physical aperture (let's call it the iris) as seen from the front of the lens - ie it is the image of the iris formed by the lens elements in front of the iris. If there are no lens elements in front of the iris, the iris and the entrance pupil are one and the same.

    What that means is that a zoom lens can have a variable entrance pupil diameter without the iris changing physical size. It may change position within the lens, as may the elements in front of it. These position changes affect the size of the image. In some lenses the iris itself may actually change diameter as the lens is zoomed as well as the magnification changing. The two effects may combine to produce a constant f-number.

    There is also an exit pupil - the image of the iris, as seen from the back of the lens. The ratio of the exit pupil diameter to the entrance pupil diameter is known as the pupil magnification.

    The entrance pupil is also the place from which the lens 'sees' the world outside - the centre of perspective of the lens. It is where the lens and camera combination should be rotated for a stitched panorama.

    There are a few threads on the issue, here is a link.

    Within that thread is the definition the ISO use:

    ISO 517:2008 Photography - Apertures and related properties pertaining to photographic lenses - Designations and measurements

    f-number
    the reciprocal value of the relative aperture

    relative aperture of a photographic lens
    twice the numerical aperture where the numerical aperture is the sine of the semi-angle subtended by the exit pupil at the focal plane
    NOTE For photographic applications, the relative aperture is equivalent (within 1/3 stop) to the ratio of the diameter of the entrance pupil to the focal length.

    entrance pupil
    image of the aperture stop as viewed from a point in the object space on its optical axis (the image of the aperture stop formed by the front elements of the lens)

    aperture stop
    physical stop that limits the cross-section of the light beam that can pass through the lens to reach the centre of the on-axis image

    The definition of relative aperture hints at the theory behind all this. The exit pupil is where the image is 'projected' from.

    Best,
    Helen
     
    Last edited: Jul 21, 2008
  12. manaheim

    manaheim Jedi Bunnywabbit Staff Member Supporting Member

    Joined:
    Sep 2, 2005
    Messages:
    14,394
    Likes Received:
    3,261
    Can others edit my Photos:
    Photos OK to edit
    och. my brain hurts.

    *manaheim bookmarks another helen post and plans to come back and read later*

    :lol:

    Thanks, Helen. :)
     

Share This Page

Search tags for this page

aperture number photo mean?

,
how did aperature number come about
,
what do aperture numbers mean
,

what do the aperture numbers mean

,

what do the aperture numbers represent

,
what does aperture number mean
,
what does f number mean
,

what does f number mean in photography

,
where do aperture numbers come from
,
where does the aperture number come from