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Old Feb 17, 2012, 1:27 PM   #11
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I don't think underexposing causes noise. A dark image just makes whatever noise that exists more apparent.
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Old Feb 17, 2012, 2:49 PM   #12
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Quote:
Originally Posted by TCav View Post
I don't think underexposing causes noise. A dark image just makes whatever noise that exists more apparent.
I haven't quite got my head round this yet but there is a suggestion that it does because the shadows contain fewer levels than the highlights so you should try and 'expose right' to minimise noise.

http://www.luminous-landscape.com/tu...se-right.shtml
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Old Feb 17, 2012, 3:06 PM   #13
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Different ways of saying the same thing. The darker areas are closer to the noise floor of the sensor, so the signal to noise ratio is poor. Exposing to the right moves the photo further away from the noise floor so the noise is relatively less. (but still visible in the darker areas) Overexposing a sky in order to show detail in a backlit subject is fairly common, and in some cases nearly impossible to avoid.

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Old Feb 17, 2012, 3:11 PM   #14
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Exactly. As VTphotog mentioned, a given sensor will have certain "noise floor".

So, if the signal from your light source is not strong enough (due to settings that lead to underexposure), you won't overcome the noise that's already present.

Underexposing an image with a digital camera is like trying to tune in a weak radio station, where you hear static, hiss and hum; only with a digital camera, you see image noise instead of hearing audio noise.
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Old Feb 17, 2012, 3:52 PM   #15
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Good analogy, JimC.

Of all the different sources of noise, the vast majority of them make pixels brighter than they should be. (There are some sources of noise that can make a pixel darker, but these are rare.) In images or portions of images that are overexposed, the noise is lost among the blown highlights. In images or portions of images that are underexposed, the noise is obvious among the dark pixels.

And to be clear, all images contain noise (pixels that are brighter than they should be), but as you increase the ISO setting, the difference between the correct pixels and the errant pixels is magnified. So noise at ISO 200 is not unusual; it's just unususal to see it.
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Old Feb 18, 2012, 1:37 AM   #16
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Not that unusual, if you look through your image library into the regions of your images that are at the left edge of the histogram and zoom to 100% you can find noise everywhere.

This is very apparent even on my 5DMkII.

Which is why in low DR scenes ETTR works so well. The best thing you can do is over-expose, just short of clipping, then pull back the exposure in post.
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