Star-shaped lights at night and the human eye

Discussion in 'Digital Photography' started by Ridge08, Mar 11, 2009.

  1. Ridge08 macrumors member

    Feb 2, 2009
    I was walking home from taking some photos one night about a month ago. Points of light in my photos had taken on star shapes (no surprise). If I understand things rightly, you get a star because the camera aperture is not perfectly round; and the number of points is determined by the number of aperture blades.

    Anyway, entering my apartment`s car park, I looked up at a street lamp and saw a many-pointed star. Then I thought about the stereotypical nativity scene, with a four-pointed North Star above baby Jesus.

    It got me wondering what creates the star effect we perceive with our eyes. Can anyone provide any illumination on this?

    With a street lamp, I guessed that the glass casing could be responsible. But what about stars in the sky? I also noticed that the stars I see emanating from given object don`t always have the same number of points.

    Another thought: the pupil in the human eye is controlled by two sets of muscles: circular, which contract to make the aperture smaller; and radial, which contract to make the aperture larger. Obviously, there`s a finite number of radial muscles. Shouldn`t that mean that the pupil isn`t perfectly round? If so, how many blades does the aperture have?

    The obvious question: is there any relation between nighttime light sources as stars and the shape of our eye`s pupil?

    Another question: as far as I know, people generally have imperfectly-symmetrical muscles all over the body. Does this apply to the eye too? I know there`s a condition where people have oval-shaped pupils. I wonder how this affects their visual perception.

    Finally, any suggestions for where can I read more about the human eye and how it relates to photography?
  2. Pikemann Urge macrumors 6502

    Jan 3, 2007
    When we see point light sources which are smaller than the diameter of the pupil, that affects the way we see them. The reason we see streaks probably has something to do with the small irregularities of the iris. Not much help, I know...

    I wish I could suggest a book or a source but I'm blank on this one.
  3. Ridge08 thread starter macrumors member

    Feb 2, 2009
    Wow, it`s quiet here!

    Pikemann, what do you mean by "sources of light smaller than the diameter of the pupil"? The pupil is only a few mm across. Clearly you`re talking about it being a function of distance. Do you remember where you read that? I`d be curious to learn more.
  4. Ryan1524 macrumors 68000


    Apr 9, 2003
    Canada GTA
    In a human eye, the iris is almost perfectly round. It's not a perfect circle, but it's not composed of a small number of diaphragms, therefore any point light source will never exhibit the same startburst pattern we see on photos.

    During night time, I only see start patterns because my eyelashes actually sits pretty low and that's what's causing the pattern. If I open my eyes wider, the picture gets clearer and the star pattern disappears altogether. The only thing I see around a light source is a strong glow. Almost like a very light fog of glowing sphere around the light, but not a star pattern.
  5. suburbiton macrumors member

    Mar 9, 2009
    Say what?! The only way it affects the way we see them is that we see them.
  6. FX120 macrumors 65816


    May 18, 2007
  7. Hmac macrumors 68020

    May 30, 2007
    Midwest USA
    The lens flare you see in your camera is from light bouncing back and forth between the various optical elements, including internal reflections within a given lens element. Optical coatings on glass lenses attempt to miminize flare, but can't eliminate it.

    Same function is at work causing lens flare in the human eye - reflected light within the tear film, cornea, and lens.
  8. marbles macrumors 68000


    Apr 30, 2008
    EU mostly
    I have that,Ovoidpupils- quite weird to look at when close up or so I've been told, not sure how or if it has affected my vision much at all though ? , one time I was told by an optician all he had to do was make a slight adjustment to the lens of any spectacles or contact lenses...anything you want to know just ask I'll help if I can...
  9. Hmac macrumors 68020

    May 30, 2007
    Midwest USA
    It's a rare condition. It doesn't affect perception or acuity, but ovoid pupils are associated with decreased ability of the pupil to change size relative to light. It doesn't have anything to do with lens flare in the human eye.
  10. dmz macrumors regular


    Jan 29, 2007
    The "glow" you see around bright lights and the sparkle you see around stars are caused partly by the atmosphere, partly by inner reflections in the optic elements, but it is also partly caused by halation - literally the halo effect, and I don't mean Apple's iPod "halo" effect!.

    The retina is a fixed-resolution device, and objects that illuminate a small area of the retina, smaller than the rod or come that is receiving the stimulus, don't resolve into a single point, and the light hitting the retina illuminates a small area about the point of contact, again, here come the internal reflections, causing "halation", which is "sensed" by nearby rods/cones, and causes a somewhat random effect of "sparkle" or "glow". And, yes, it happens with larger objects too!

    Hey, this happens with digital sensors, film, photo paper, anything that receives an image photomechanically, and we are so used to seeing it that an image without it looks fake or "wrong" somehow.

    You can't even trust you eyes!


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