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I am fairly short-sighted and wear glasses pretty much all the time. Naively, I would expect that when I take my glasses off, the image I see should look very much the same as as a photograph that's out of focus, or an image to which someone has applied a Gaussian blur filter. In bright conditions this is mostly true, but at night if I look at distant point sources of light, the blurry halos around them appear to have quite a complex structure, and a fairly well-defined edge. Why is this?

To make it clearer what I mean, and at the same time to make a hypothesis about the answer, here is a public domain image of some lights, taken with a camera out of focus:

enter image description here

(image source.) Notice that the image cast by each light is not simply a uniform circle. They are each slightly hexagonal; they are slightly brighter around the edge than in the middle; and there are distinct nonuniformities in the brightness in the interior of each shape.

What I see is actually fairly similar to this but much more extreme. The shape formed by a light is much further from a circle, being very irregular in shape and distinctly elongated in one direction. (This latter is perhaps not surprising given that I have a strong astigmatism.) The bright edge to the shape, and the non-uniformities in its brightness, are much more pronounced than in the camera image.

Now, in the photograph I know that the hexagonal shape is due to the hexagonal aperture in the camera. This can be ruled out in my eyes because my pupils are definitely circular. (Unless some light also passes through parts of the iris?) I would guess that the bright edges are caused by diffraction, and that the nonuniformities are caused by defects in the lens and/or dirt on its surface. I suppose that diffraction and lens aberrations must be the causes of what I see when looking at distant lights without my glasses.

So in the end, my question is mostly just, am I right about this? Are the shapes I see the result of defects in my eye's optics, and which specific anatomical features of the eye are likely to be their main causes?

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  • $\begingroup$ This isn't an answer, just speculation. In dim light, your rods are primarily responsible for perception (cones don't work very well). Cones are also more concentrated in the center of the retina, which would mean that rods are more concentrated at the edges. Something to do with focusing in nearsighted people might explain this. $\endgroup$
    – blep
    Apr 6, 2013 at 15:51
  • $\begingroup$ dd3's suggestion makes sense to me but I wonder if you could add a short paragraph to explain the 'structure' a little more? $\endgroup$
    – daniel
    Mar 14, 2014 at 19:35
  • $\begingroup$ @daniel I'm not sure if I can easily describe it in words. If I get a chance some time I will make a sketch. $\endgroup$
    – N. Virgo
    Mar 15, 2014 at 2:17
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    $\begingroup$ @WYSIWYG I think approximately 50% of the population will not have any difficulty knowing what I perceive. However, I will edit the post to make it clearer. $\endgroup$
    – N. Virgo
    Dec 24, 2014 at 13:31
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    $\begingroup$ @WYSIWYG see the edit. In making it clearer I may have answered most of it myself, but I hope you can understand what I mean now. I've never spoken about it to an opthalmologist, because it's a subtlety of perception that only occurs at night, so I've never thought to mention it. (I suspect that it's what everyone shortsighted sees.) $\endgroup$
    – N. Virgo
    Dec 24, 2014 at 13:48

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This phenomenon is known as bokeh.

enter image description here

You are right in the hexagonal shape being produced by the iris (of the camera), and from this, it is clear why this happens: The iris blocks out some of the light travelling into it, and a shape approximating that of the camera's iris aperture is projected onto the CCD.

The same thing happens for the eye. The iris will block out a portion of the light, causing the bokeh to approximate the shape of the iris aperture (aka the pupil), which is roughly circular for humans. Animals with non-circular pupils will perceive differently shaped bokeh. If the lens suffers from astigmatism, it will also result in non-spherical bokeh being projected onto the retina.

This stackexchange link goes into greater detail on Gaussian blur vs bokeh.

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  • $\begingroup$ It seems like you're saying that the shape of the human pupil is hexagonal? If so, I would like to see a reference for this. $\endgroup$
    – fileunderwater
    Jan 20, 2015 at 14:24
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    $\begingroup$ @fileunderwater No, I am not. The hexagonal shape of the bokeh in the OP was caused by the hexagonal shape of the camera iris. I will reword the answer to prevent this misunderstanding. $\endgroup$
    – March Ho
    Jan 20, 2015 at 17:15

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