Alley in Pisa, Italy

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Why does the signal to noise ratio decrease when using high ISO speeds (e.g 3200) when shooting at night with a digital camera?

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I suppose you wanted to say 3200 ISO, as 320 is not that high! :) –  LudoMC Jan 29 '11 at 15:50
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Welcome to photo.SE! This is very similar to several other questions related to ISO; try photo.stackexchange.com/questions/70/… or search for "iso". –  Reid Jan 29 '11 at 16:08
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possible duplicate of What is "ISO" on a digital camera? –  mattdm Jan 29 '11 at 16:37
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3 Answers 3

With a digital camera, upping the ISO rating is achieved by aplifying the signal from the sensor. There will always be some random noise, and upping the ISO aplifies this, to the extent that it can start to become noticeable.

Different cameras behave differently in this respect, with the general case that a larger, more modern sensor will have better results then an older or smaller sensor (So a full frame DSLR like the Canon 5D mkII will outperfrom an older Canon 20D or a smaller Canon IXUS)

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And note that sensors designed for dark-field photography (the sort of thing an astronomer would attach to a telescope) are actively cooled (usually by a Peltier device) to eliminate thermal noise that won't be hidden by a light-induced signal. As far as I know, only multishot and scanning camera backs come equipped that way in the world of general photography. –  user2719 Jan 30 '11 at 0:52
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When you increase ISO you effectively increase amplification of the signal from sensor. There is always some noise present in signal, so when you amplify signal you also amplify this noise.

I'm not sure, but i think amplification process itself may introduce more noise.

Also at night there is little light, so signal to noise ratio is worse - so noise is going to be more noticeable.

As Rowland Shaw stated above sensors differ in they ability to capture light and dealing with noise. In general larger sensors tend to perform better - they usually have larger pixels, which means each pixel gets more light. Manufacturers keep working on sensor technology so newer sensors also tend to perform better than older ones.

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The fact that it's night adds nothing to the noise. High ISO at daylight will give the same noise. Low light is merely a reason for selecting high ISO setting. –  ziggystar Jun 23 '12 at 19:43
    
When using high ISO in daylight the signal (the light entering the camera) is much stronger than at night. If the signal is many orders of magnitude stronger and the noise level is the same, you have a better signal to noise ratio in daylight at the same ISO. The longer shutter speeds generally used at night can also have an effect as some of the extra heat generated winds up as noise. –  Michael Clark Feb 19 '13 at 0:02
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The Signal to Noise Ratio (SNR) does not decrease when the ISO setting is increased. SNR gets worse because there is less light around at night, and therefore less signal. Also noise will be higher if you use a longer exposure due to dark current and other electrical sources.

The reason to increase ISO is to boost the signal before digitizing to prevent even more noise.

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You want to, you know you do.....I sense the book pictures coming out ;) –  rfusca Jun 23 '12 at 23:48
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