Pixel density - can the playing field be leveled???

Started Jun 6, 2009 | Discussions thread
ejmartin Veteran Member • Posts: 6,274
Re: Per area read noise?

Yes, I think that I understand that. The read noise must scale with the pixel density the way you describe, in order to keep it at the same relative level compared to the shot noise, and assuming that the read noise is scaled like that, and then comparing a 10mp G11 to a hypothetical 20mp G11, won't the 'image level' (or per area) noise then simply be the sum of the noises (shot/read combined) from the individual pixels, resulting in 41% higher per area noise for the 20mp camera? (at its higher 'spatial frequency' or resolution)

Only for read noise. There might also be a slight further increase in read noise, due to the fact that the processor is handling double the data rate, and faster readout also tends to increase noise (this is the reason for the highly parallel readout in the Sony Exmor CMOS chips in their DSLR's, and why they have such good read noise performance at low ISO).

Since read noise R is fixed, shot noise increases with signal as sqrt[S], and noises add as RMS, total noise is N=sqrt[R^2+S] when all are measured in electrons equivalent. At high enough signal the noise is shot noise dominated, and tends to be independent of pixel count. It is for low signal (shadows at low ISO, or most of high ISO) that read noise effects become important.

Thus in the noise/resolution tradeoff, it is relevant what sort of photography you do; landscape photographers would want high resolution for low ISO shooting, indoor photographers might appreciate better high ISO performance.

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