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<blockquote data-quote="pforsell" data-source="post: 493095" data-attributes="member: 7240">You ignore read noise which does not depend on the signal. Different noises add in quadrature. As a matter of fact read noise dominates in the shadows where signal is weak and thus photon shot noise is low. This is the photon shot noise, yes. In the same time there's always read noise present. With Nikon D3X the read noise scales with ISO as follows:ISO 100 -&gt; read noise 6.0 electronsISO 200 -&gt; read noise 4.7 electronsISO 400 -&gt; read noise 4.2 electronsISO 800 -&gt; read noise 3.7 electronsISO 1600 -&gt; read noise 3.2 electrons (minimum)HI +1 (3200) -&gt; read noise 3.8 electronsHI +2 (ISO 6400) read noise 3.8 electronsIn your scenario (very low light) adding the shot noise (S.N.) and read noise (R.N.) in quadrature delivers total noise for example at ISO 100 and ISO 1600 as follows:ISO 100: S.N. 10.0 + R.N. 6.0 -&gt; total noise 11.7ISO 1600: S.N. 10.0 + R.N. 3.2 -&gt; total noise 10.5The ISO setting in most cameras is not only digital scaling of the electron counts, but also analog amplification of the signal. Only D7000 and D2X to my knowledge use purely digital scaling without an analog front end.I read your posts in the &quot;Before and After&quot; thread <a href="http://nikonites.com/post-processing/28233-post-your-before-after-pictures.html#axzz3ltQleuo5" target="_blank">http://nikonites.com/post-processing/28233-post-your-before-after-pictures.html#axzz3ltQleuo5</a> and you are already halfway there. Please do not turn back now, but follow the path to where it leads.</blockquote>

[QUOTE="pforsell, post: 493095, member: 7240"] You ignore read noise which does not depend on the signal. Different noises add in quadrature. As a matter of fact read noise dominates in the shadows where signal is weak and thus photon shot noise is low. This is the photon shot noise, yes. In the same time there's always read noise present. With Nikon D3X the read noise scales with ISO as follows: ISO 100 -> read noise 6.0 electrons ISO 200 -> read noise 4.7 electrons ISO 400 -> read noise 4.2 electrons ISO 800 -> read noise 3.7 electrons ISO 1600 -> read noise 3.2 electrons (minimum) HI +1 (3200) -> read noise 3.8 electrons HI +2 (ISO 6400) read noise 3.8 electrons In your scenario (very low light) adding the shot noise (S.N.) and read noise (R.N.) in quadrature delivers total noise for example at ISO 100 and ISO 1600 as follows: ISO 100: S.N. 10.0 + R.N. 6.0 -> total noise 11.7 ISO 1600: S.N. 10.0 + R.N. 3.2 -> total noise 10.5 The ISO setting in most cameras is not only digital scaling of the electron counts, but also analog amplification of the signal. Only D7000 and D2X to my knowledge use purely digital scaling without an analog front end. I read your posts in the "Before and After" thread [URL]http://nikonites.com/post-processing/28233-post-your-before-after-pictures.html#axzz3ltQleuo5[/URL] and you are already halfway there. Please do not turn back now, but follow the path to where it leads. [/QUOTE]

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