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Why flash of light in a digital camera?

  1. Dec 7, 2006 #1
    When taking photos with a digital camera there is usually a burst of light before the photon is taken. Why is this? Is it because there is usually not enough light reflecting off the object being photographed. It is this reflected light that is what is being recorded on the film inside.
     
  2. jcsd
  3. Dec 7, 2006 #2
    As yourself what exactly is the photograph taken of? What is everything you can see (trivially speaking of course)? Is there times when you do not use a flash when taking a photo?
     
  4. Dec 7, 2006 #3

    turbo

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    Many times, photos are taken of people. The pre-flash has a physiological effect on them. Can you figure out what it is?
     
  5. Dec 7, 2006 #4
    a physiologic pre-flash effect: my guess is a smile... or something of the sort if that is what you mean...
    Anyways i do not see what is the relation between flashes and digital cameras. The idea is to illuminate better to better expose a photo or capture fastly moving objects in an "instantaneous" manner (1/1000 sec).Xenon gas is used mainly .
    However what i think you really mean is the "red eye effect", the retina of the human eye reflects red light straight back in the direction it came from, pictures taken from straight in front of a face often exhibit this effect. It can be somewhat reduced by using the "red eye reduction" found on many cameras (a pre-flash that makes the subject's irises contract). However, really good results can be obtained only with a flash unit that is separated from the camera, sufficiently far from the optical axis, or by using bounce flash, where the flash head is angled to bounce light of a wall, ceiling or reflector.
     
    Last edited: Dec 7, 2006
  6. Dec 7, 2006 #5

    turbo

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    Another clue. The pre-flash is a feature that can be turned off, but is often selected by default so that your pictures of people will be more attractive (and no, it doesn't make them smile).
     
  7. Dec 7, 2006 #6

    turbo

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    Give khalil a prize. The reason that pre-flash is prevalent on digital cameras is that most of them are very small all-in-one units, meaning that the flash is very close to the lens so the light bounces straight back from the subjects' eyes, giving a really bad case of red-eye. Most of these are designed for snapshots (point and shoot) and are not aimed at the part of the market that would understand or use bounce flash even if it were available. Also, bounce flash can be very hard on battery life - not a good thing in a pocket-sized camera that uses AA batteries and has to use battery power for auto-focus, exposure control, image capture, etc. If you double the path-length of the light from the flash, you have to quadruple the intensity of the flash to achieve the same level of illumination assuming you are bouncing of a perfectly reflective surface.
     
  8. Dec 7, 2006 #7
    How does the red light get into the eye in the first place (assuming no flash is used)? The camera without the flash is a nonluminous object so the only light must be light reflected off the camera and goes straight into the eye. Does the eye only reflect red and nothing else?
     
  9. Dec 7, 2006 #8

    chroot

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    If you use a bright light to look inside someone's eye -- as they do in the eye doctor's office -- you'll see that the inside of the eye looks red. (So yes indeed, it reflects mostly red light and absorbs the rest.)

    The same principle is what causes red-eye in photographs. Bright light, reflection.

    - Warren
     
  10. Dec 7, 2006 #9
    flash and pre-flash? So there are usually two flashes when a photo is taken, one very quickly in succession of the other.

    So all cameras must have a flash to illuminate the object being photographed but the retina reflects red light only so the flash causes red eyeness. Therefore a pre-flash is equiped to get rid of this feature by narrowing the iris where light can get through.
     
  11. Dec 7, 2006 #10

    turbo

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    That's it, essentially. The retina has more blood vessels per unit are than any other part of your body, and so it looks very red when illuminated.
     
  12. Dec 7, 2006 #11

    chroot

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    Note that the shutter of the camera is only open for the last hurrah -- the biggest and brightest and last flash. The other pre-flashes are done with the shutter closed, and have no purpose other than eliminating red-eye.

    - Warren
     
  13. Dec 7, 2006 #12
    With modern cameras is the pre flash and flash done with the same light globe?
     
  14. Dec 7, 2006 #13

    turbo

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    Yes. I'm not sure if the strategy is to keep the pre-flash intensity at a lower level or perhaps to keep their duration very short.
     
  15. Dec 7, 2006 #14

    jtbell

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    When my camera is set to red-eye reduction mode, I actually get three flashes! I think the first one is used to help the camera set the exposure automatically (by measuring the reflected light from the scene), the second one narrows the subject's eye pupils to reduce the red-eye, and third one illuminates the scene when the shutter opens.
     
  16. Dec 7, 2006 #15
    What would be the point of the first flash? To get a record so that how much light is needed for the third flash would be known? It would be desirable to keep the third flash as low as possible wouldn't it as the iris won't be completely shut. Also you wouldn't want the intensity of the second flash too be so high that the person being photographed blinks. Although when that person does blink would the photo have already been taken?
     
  17. Dec 7, 2006 #16
    some of your questions answers may vary with brand and camera type. a profesional-film using camera, pocket-sized digital camera, and a camera-phone would all flash in differnt styles and for different puposes.

    do we have anyone on here who works with a specific camera company.?
     
  18. Dec 8, 2006 #17
    just a suggestion ...three flashes ...maybe it has to do with retinal impression
     
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