Aperature - how does it work?

Discussion in 'Photography' started by Ed E., Oct 10, 2003.

  1. Ed E.

    Ed E. Guest

    Ok, this sounds basic but I just don't understand yet.

    I understand that aperture controls how much light comes through a lens. My
    questions are: 1> How can the amount of light be reduced without blocking
    the edges of the image and 2> Why does changing the aperture affect the
    depth of field?

    Thanks for helping to clear up this camera mystery for me....
     
    Ed E., Oct 10, 2003
    #1
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  2. Ed E.

    Don Stauffer Guest

    Near the center of the lens, the aperture is not chopping off the
    'edges' of the image. Rays from every part of the object pass through
    the whole lens. Restricting the area of the aperture controls how MANY
    from each point in the object pass through the lens, and each
    contributes its bit of brightness. You need to check a photography book
    that shows some rays traced through the lens to see how this works.

    As far as depth of field, you need to find an explanation of how a
    pinhole aperture works. Both this and the effect of the aperture itself
    need to show some geometry, i.e, trace some rays, so it is almost
    impossible to describe entirely with words.
     
    Don Stauffer, Oct 10, 2003
    #2
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  3. I'm going to attempt to describe it with words.

    Let's start by thinking of a single point on the scene you are
    photographing. Think of light rays leaving that point and striking the
    lens. Think of the lens as a circular disk. So, the whole set of light
    rays from that point to the lens forms a cone. Now think about the
    other side of the lens. The lens bends the light rays so that they all
    converge to a point on the film. That will be another cone, but a short
    one. It is from the lens to the point on the film. Now, let us throw
    the camera out of focus by moving the film a tiny bit toward the lens.
    What had been a point on the film where the cone ended now becomes a
    small circle. If you now reduce the lens aperture by a diaphragm the
    cuts the apparent lens diametert in half, you see that the cone of light
    becomes a narrower cone, and the small circle at the film becomes
    smaller, in fact half its previous size.

    A circle like that is called a "circle of confusiion." If the circle is
    small enough, your eye looking at a picture cannot distinguish it from a
    point. So, the slightly out of focus image appears to be in focus.
     
    Herbert Kanner, Oct 10, 2003
    #3
  4. Ed E.

    EktarEd Guest

    The image does not go through he lens parallel to the sides of the lens. It is
    focused through the center of the lens. If it were not for this fact then why
    would you need a lens in the first place. Just a plain old hole in the front of
    the box should suffice. The camera obscura was simply this, but the plain old
    hole was very small, thus causing the light to focus in a plain as it were.

    Have to run , ,but pick up a book on depth of field, and get a kick out of
    Bokeh.
     
    EktarEd, Oct 10, 2003
    #4
  5. I tend to think of DoF in the human eye. If you squint you tend to see
    farther objects more in focus, as well as everything in your view. If you
    widen your eye the close objects will be oin view, but the farther objects
    will lose definition.

    Not sure if this answers the question, but it sure keeps higher numbers,
    smaller aperture straight for me.
     
    Trevor Graham, Oct 10, 2003
    #5
  6. Ed E.

    chad Guest

     
    chad, Oct 11, 2003
    #6
  7. Ed E.

    mike II Guest

    That may be due to a deformation (re-formation?) of the 'lens' by the
    muscular pressure of the eyelids.


    mike

    --
    __ __ __ __ __ __ __ __
    / /\ / /\ / /\ / /\ / /\ / /\ / /\ / /
    / /\ \/ /\ \/ /\ \/ /
    /_/ \/_/ \/_/ \/_/ \/_/ \/_/ \/_/ \/_/

    ..let the cat out to reply..
     
    mike II, Oct 11, 2003
    #7
  8. Ed E.

    Alan Browne Guest

    no, same effect as a pinhole.
     
    Alan Browne, Oct 11, 2003
    #8
  9. Ed E.

    Charlie D Guest

    Yep.
    Even better than that, squeeze the forefinger of each hand against the
    thumb the same hand so the tips are even. Then bring your two hands
    together and squeeze the four fingers together so you have a 1/8" square
    hole between them.

    If you could decipher my instructions, hold them close to your eye and
    look through the hole.
    If you forget your reading glasses it'll allow you to read a menu in a
    PINCH. Ha Ha!
     
    Charlie D, Oct 11, 2003
    #9
  10. I sugeest you look at a book such as the Focal Encyclopeadia of
    Photography. What you're asking for here requires extensive education
    in the physics of photography. That's what such books are for. Go to a
    good library and look through the photo section for such books.
     
    Michael Scarpitti, Oct 11, 2003
    #10
  11. Ed E.

    Alan Browne Guest


    Understanding how aperture affect DOF requires a couple diagrams to
    illustrate it ... it doesn't require "extensive education in the physics
    of photography."
     
    Alan Browne, Oct 11, 2003
    #11
  12. If you do this, be sure to choose the carrot soup.....The vitamin A is good
    for your vision........
     
    William Graham, Oct 11, 2003
    #12
  13. Ed E.

    Mxsmanic Guest

    The light that reaches each point on the film consists of many rays that
    follow different paths through the lens from the same point in the
    original scene. In fact, for any given point, there are rays reaching
    that point that pass through every part of the lens.

    Closing the aperture does cut off some of these rays around the edges,
    but other rays that pass through the more central portion of the lens
    still get through. The net result is that each point on the film
    receives less light from the corresponding point in the original scene
    as the aperture is closed ... but all points continue to receive light.
    When a lens is focused at a specific distance, light from points at that
    distance is focused to sharp points on the film. However, lights from
    points in front of that distance, or behind it, are focused in front of
    the film, or behind the film. These latter points actually form small
    discs where they intersect with the film, instead of sharp points, which
    is why details that are not at exactly the focus distance are always a
    bit blurred.

    These discs are actually projections of the full cylinder of light
    passing through the lens. When the lens is wide open, the discs are
    formed of light passing through the lens at all angles from the original
    scene, and so the discs are large. When the aperture is closed down,
    the discs shrink.

    Now, depth of field is an optical illusion, caused by the fact that the
    human eye cannot tell the difference between a disc and a sharp point if
    the disc is very small. So objects at distances that are not at
    precisely but focus distance but still close to it project very small
    discs onto the film, and this creates the illusion that the
    corresponding scene details are still in focus. Since closing the
    aperture makes all the discs smaller, the depth of field--the range of
    distances outside the actual focus distanct at which the discs are small
    enough to look like sharp points--expands as the aperture gets smaller.
    Thus, when you close the aperture, the depth of field increases.
     
    Mxsmanic, Oct 11, 2003
    #13
  14. I never thought of that! That's great! :)
     
    drhowarddrfinedrhoward, Oct 11, 2003
    #14
  15. Ed E.

    Deathwalker Guest

    when applying pressure to your eye will compress it. This will make the eye
    itself slightly egg shaped or rugby ball shaped. This means distance
    between cornera and lens has changed hence focal length. Also the muscle
    controlling the lense stretch or relax the gelatinous lens which means it
    gets fatter or thinner which means the higher or lower the refraction.
    Squinting forces lens muscles towards the centre hence it becomes fatter.

    As for depth of field. The camera only uses the centre portion as mentioned
    before. The wider the aperture the greater the quality the lens has to be
    because you are using a greater area which has to be distortion free. If
    memory serves this is called covering power. Which is why zooms with
    apertures of 2.8 are so expensive. You usually find zooms that have
    3.5-5.6. Not only is the maximum aperture not very big but the longer the
    focal length smaller the maximum aperture becomes. A whole new set of
    problems occur with a zoom that has a continuous maximum aperture. your
    assumption of small diaphragm is correct with old machinery. It is called
    vignetting.
     
    Deathwalker, Oct 11, 2003
    #15
  16. Ed E.

    Deathwalker Guest

    War propaganda. The real reason brits were shooting down gerry's in the
    dark was cos they had developed radar in the nose of planes. The carrot
    thing was subterfuge.
     
    Deathwalker, Oct 11, 2003
    #16

  17. Well, perhaps that's overstating it, but of course the diagrams are
    hard to do here, and a visit to the library will provide a more useful
    answer than he can get here.
     
    Michael Scarpitti, Oct 12, 2003
    #17
  18. All of the above may be true, but it is also the case that looking through a
    small opening, like a pinhole will enable you to see things better if you
    are nearsighted, and don't have your glasses with you, and there is enough
    light available.....This is true without touching your eye at all.....
     
    William Graham, Oct 12, 2003
    #18
  19. I can believe that....I have been eating carrots all of my life, and my
    night vision has always been lousy, and has not improved as a
    result.....Just another urban myth/old wives tale.......
     
    William Graham, Oct 12, 2003
    #19
  20. Ed E.

    james Guest

    Interesting you should mention the thing about looking thru a pinhole. I
    had a major eye injury three years ago and ended up being legally blind in
    my left eye. My Cornea is fogged from chemical burns and invasive blood
    veins that grew into it to try and heal it. Whenever I go for a checkup one
    of the things they do to check my vision is have me try and read an eye
    chart with my left eye, looking thru pinholes.
    I can actually see 20/400 to 20/800 (on bad days) this way. Otherwise, the
    chart's largest letter (single E) is a complete blur. I have messed with
    different size of pinholes to see if I could improve my vision in my left
    eye and the smaller (up to a point) the better. But, it does not translate
    into usable vision for both eyes having one seeing only thru a pinhole and
    the other thru regular glasses lense.
    Just thought I'd pop in here and mention that looking thru a pinhole, really
    can improve vision up to a point.
    james
     
    james, Oct 12, 2003
    #20
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