How Is Noise Created?

Discussion in 'Beyond the Basics' started by Buszaj, Jun 3, 2008.

  1. Buszaj

    Buszaj TPF Noob!

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    Well, most of us know that when you increase the ISO, you get more noise. I was wondering, why does this actually happen?

    Thanks
     
  2. Alex_B

    Alex_B No longer a newbie, moving up!

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    when a signal is created from a sensor pixel, then there is always noise.

    This noise depends on the pixel, on its temperature and whatever.


    If we now have a weak signal since there is not much light, then we have a bad signal to noise ratio (relatively small signal, but lots of noise). If we have more light, then that ratio becomes better, since the signal increases but the noise remains more or less the same.

    If we now increase the ISO, this only means that we amplify more, hence both signal and noise are amplified. With a bad signal to noise ratio we need to amplify more (higher ISO) to get the same signal strength. But more amplification then means more noise since the signal to noise ration remains the same.
     
  3. Alex_B

    Alex_B No longer a newbie, moving up!

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    But to my knowledge the acronym CA usually refers to chromatic aberration. This is only related to the lens/ the optical system, and not to the ISO.
     
  4. ann

    ann No longer a newbie, moving up!

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    it is my understanding it is the increase in the electical charge that has to be increase to enable the sensor to record the image at the higher ISO's.

    for instance, with film, the size of the silver particles are increased in size to make the film more light sensitive, which translates into grain.

    with a sensor, there is no way to change the size to make it more sensitive so the electrical impulse that records the images is boosted, which ends up as noise (grain)

    there may be a more complicated answer and i am sure somone here can expand on my answer.
     
  5. Socrates

    Socrates TPF Noob!

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    How about a more simplified answer?
    Think of audio and, specifically, the background sound (noise) in your environment. The louder it is, the louder you must shout to overcome it. Just as audio noise exists everywhere on earth to greater or lesser degrees, so does electrical noise. Just like sound, the greater the electrical noise, the greater the signal must be to overcome the noise. Hence, the S/N (Signal-to-Noise) ratio that Alex mentioned.
     
  6. Buszaj

    Buszaj TPF Noob!

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    Thanks for the answers, I get it now.

    Ohhhh, I see. Learn something new everyday....I said that because I heard that noise is also referred to as CA, but I guess ISO noise isn't the same.
     
  7. JerryPH

    JerryPH No longer a newbie, moving up!

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    Just wanted to mention... CA (chromatic aberration) is an undesirable lens effect, not high ISO noise related.
     
  8. ann

    ann No longer a newbie, moving up!

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    socrates i thought that was a simple answer, i thought of the audio example but it wasn't along the same lines you gave, which certainly makes sense as well.
     
  9. Socrates

    Socrates TPF Noob!

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    :hug::
     
  10. cdanddvdpublisher

    cdanddvdpublisher TPF Noob!

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    that's a great explanation
     
  11. Socrates

    Socrates TPF Noob!

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    Thanks.
     
  12. Bifurcator

    Bifurcator TPF Noob!

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    I guess we're talking about grain type noise?

    I always thought it was that the sensitivity of each photocite (photo sensitive cell) is different from one to the other. When you amplify the signal coming from them (by increasing the ISO) you are amplifying the differences causing one cell to register very high and one to be normal (average) or too low.

    Consider the simple experiment.

    Givens:
    • The light entering the lens = 3
    • A recording of 0 registers as black (IRE 0)
    • A recording of 32 registers as 100% white.
    • You have a 3 pixel CCD :)
    • For this example let's pretend that an ISO amplification of 100 is 1x, an ISO amplification of 200 is 2x and so on.
    • You take 3 sample test shots at ISO 100, 400, 800.

    At ISO 100 the three photocells register 2,4,2 respectively (one photocell per pixel). As seen in a grey scale image there's very little difference between the three pixels.

    At ISO 400 the three photocell signals are recorded as 8, 16, 8 respectively. Now the difference is very noticeable when viewed in the grey scale image.

    At ISO 800 the recording is 16, 32, 16. The relative difference in visual perception when viewing a gray scale image is also high enough here to be called noisy.

    The CMOS process produces generally more sensitive cells (faster) and with greater grid uniformity so less noise.

    The non-uniformity of light as particles plays a roll here too (where one cell may receive more or less than the average number of photons in a given source) but I always thought it was a minor factor compared to the surface imperfections that form the photosensitive substrate, their electrical response variation, the optical inequalities from one micro-lens to the next, and the roll that amplification plays - as explained above.

    No?
     

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