Quote Originally Posted by Jon Ruyle
I would have guessed read noise would be larger when you have more pixles (so high pixel density would hurt snr).
You're in good company. That's the commonly accepted viewpoint among most photographers, web sites, magazines, and just about anywhere you look. But I still think it's incorrect, and it persists due to a fundamental misunderstanding of scale: chiefly, that pixels are compared at 100% crop, where smaller pixels are unfairly examined at higher magnifications and higher spatial frequencies. If all pixel sizes were examined at the same magnification, same print size, same resolution, and same spatial frequency, the bias against small pixels would never have gotten off the ground.

Generally, I think read noise is not correlated to pixel size. For example, the Panasonic LX3 has a read noise of 5.6 electrons per pixel (2.55 ADU), compared to 23.5 electrons in the 5D2, both at base ISO. But that's comparing very different spatial frequencies. After resampling the small LX3 pixels to the much larger 5D2 pixels, read noise per output pixel goes down significantly, because random noise adds in quadrature, so the LX3 is even better. This can be tested by measuring noise on any raw image, then resampling with a good algorithm (e.g. lanczos; not the poorly implemented algorithms in photoshop), and measure the read noise again.