Question about aperture...

Discussion in 'Open Discussion' started by stw, Oct 4, 2013.

  1. stw

    stw Mu-43 Regular

    Jul 23, 2013
    I am trying to understand some aspects of aperture and how it relates to the sensor. When the aperture is wide open, I can visualize that the sensor is fully covered in light. As the aperture is stopped down, however, I am picturing that the light coming through covers less of the sensor. I am guessing that it does not happen that way, because you'd have a big area of dark/shadow on the sensor.

    In my head, I am picturing the aperture stopped down is something similar to a pinhole in a sheet of paper. Hold the paper up in front of a wall, shine a light through it, and you'll see just a small light dot on the wall. On that same sheet of paper, cut hole the size of a nickel. Hold the paper the same distance from a wall as before, then shine a light through that, and you'll see a much larger area of light on the wall.

    What am I missing? What enables the sensor to 'see' the entire picture as the aperture is stopped down?
  2. slothead

    slothead Mu-43 All-Pro

    Aug 14, 2012
    Frederick, MD
    The final lens elements closest to the sensor.
  3. Just Jim

    Just Jim Mu-43 Top Veteran

    Oct 20, 2011
    Imagine light coming through a tube it's channeled in at the front element, choked at the aperture, spit out and flipped to the sensor. The smaller the choke point (the aperture) the larger the image circle put onto the sensor.

    sensor >|< lens small aperture, larger image circle

    =|< large aperture, smaller image circle
  4. flash

    flash Mu-43 Hall of Famer

    Apr 29, 2010
    1 hour from Sydney Australia.
    You're assuming that light travels straight down the barrel of the lens, which it doesn't. Light can travel through the hole you made in your paper (and a lens) at an angle. Right up to nearly 90 degrees to the axis of the hole in the case of the paper. The size of the hole doesn't make a difference (as long as it's bigger than 1 wavelength of light). As long as light can travel through it, it can travel at an angle. To vary the angle of light coming through the hole turn it into a tube (focal length). To vary the intensity of the light coming through the hole make it bigger or smaller (aperture). As the light travels through the hole it wil be flipped and reversed (nodal point).

    If you took the peice of paper (it would need to be opaque black card), as in your example and put it in a box, you'd have a pinhole camera. There is still an image being projected threough the hole. The reason you can't see it and you see a larger or smaller white circle is because you have made ethe hole too big (overexposure), and you don't have a good viewing platform (dark enclosed space).

  5. MajorMagee

    MajorMagee Mu-43 All-Pro

    Feb 24, 2011
    Dayton, OH
    If you want to follow the pin hole analogy over to a lens then you have to think of it not as a bigger hole, but as a multitude of small holes that add up to the area of the bigger hole. These many pin holes are aligned by the lens optics to all come together at a single focal point. One small pin hole only delivers a small number of photons. Many small pin holes add up to deliver a lot of light at the focal point.
  6. b_rubenstein

    b_rubenstein Mu-43 All-Pro

    Mar 20, 2012
    Melbourne, FL
    The size of the image circle covers the entire sensor regardless of the aperture. If the size of the image circle varied with aperture, then the size of the subject would change depending on the aperture. Do you think that you've ever observed that? The aperture setting changes the amount of light reaching the image capture media.
  7. ThomD

    ThomD Mu-43 Top Veteran

    Jun 1, 2013
    SF Bay Area
    This is a great question and I'm not sure I can come up with a coherent answer, but I'm going to try anyway.

    Short answer: optics and the position of the aperture blades in the light path.

    With a straight tube, putting an aperture control at one end decreases the image circle. In a lens, the light path is controlled. When the aperture is made smaller that doesn't change how the optics are setup to project the image. Thinking of a projection helps I think.

    Wikipedia has some good articles on aperture and depth of field. From the DoF article, this image does a good job showing how a smaller opening "increases the depth of field". What is really does is block the light rays that focused on a different plane.


    Different lens designs produce different image circle sizes.For example, lens for 35mm produce a larger image circle than m43 lenses so that when you use an adapted lens are you only using the center portion of the image circle.
  8. Talanis

    Talanis Mu-43 Top Veteran

    Oct 15, 2012
    Sherbrooke, Canada
    Eric Cote
    When you squint, do you see a smaller image? No, there is just less light coming into your eyes. You also get more depth of field: it's natural to squint to see a sign that is far away.

    Sent from my Nexus 7 using Mu-43 mobile app
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