# Let's finally resolve the D800 downsampling / NR questions

Started Mar 12, 2012 | Discussions thread
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 Let's finally resolve the D800 downsampling / NR questions Mar 12, 2012

I say D800 but the question really applies to any comparison involving a higher density vs lower density sensor.

I had planned to lay all my questions out on the table in this OP but I think it's more productive to start with a focused question first and let it branch organically form there.

I'd like to start with two theorems that seem incongruent to me. Take two theoretical full-frame sensors of equal technology, one is 36MP and the other 12MP (I know the D800 has better technology vs D700 but let's table that for now):

Theorem #1 - The 36MP sensor will have higher per-pixel noise than the 12MP sensor but both will have the equal amounts of noise on a per-area basis. In other words, if you combine multiple 36MP pixels into units roughly equal in size to 12MP pixels, the amount of noise in those combined pixels will be equivalent to the 12MP pixels. This seems pretty obvious and straightforward.

Theorem #2 - Downsampling does not increase the SNR of the image, at least across all spatial frequencies within the image. Based on my understanding of previous threads on this topic*, ejmartin, John Sheehy, and others share the following opinion: Downsampling does reduce the amount of noise as measured via std. deviation, but it also reduces the amount of detail, in varying degrees across the frequency/resolution domain of that signal, so the net effect is a reduction of both detail and noise, with potentially no reduction in the SNR for certain frequencies.

These above theorems seem incongruent to me. If the 36MP sensor has the same per-area noise as the 12MP sensor but the only way to realize that equivalence is by combining the 36MP pixels into units of equal in area to the 12MP pixels (Theorem #1), but the method used to combine those pixels (post-demosaic downsampling) results in no SNR gain (Theorem #2), how in practice can a user of the 36MP sensor achieve the same SNR performance of the 12MP sensor for equal size prints?

http://www.dxomark.com/index.php/Publications/DxOMark-Insights/Detailed-computation-of-DxOMark-Sensor-normalization

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