Anyone else plan on buying Sony's new FF?

Started Jun 25, 2012 | Discussions thread
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Re: Big pixels vs small pixels
In reply to Robsphoto, Jun 26, 2012

Robsphoto wrote:

We have been told on the forums that, if you DOWNSIZE a low-light 24mp A77 image to 16mp, it THEN has much the same noise as a comparable image from the 16mp Sony A55.

Per pixel, yes. The image as a whole already has approximately as much noise as long as the sensor is from the same technology level and the same size. Any visible difference in noise is from different algorithms used to process the image in camera (such as Pentax on some models loving to smear away detail to increase ISO smoothness) or a (perhaps) difference in active light gathering surface.

Because downsampling is said to reduce noise, then doesn't it follow that a full-sized UNSCALED 24mp A77 low-light image must have more noise than a similar image from the A55?

Again per pixel. There is no indication the noise of the whole image will be any different. It may be, because there may be different trade-offs in algorithms and effective ISO choices, but there may also not be.

Because both the A55 and the A77 have the same sized APS-C sensor (23.5mm x 15.6mm), then if the smaller pixel size of the A77 doesn't affect low-light performance at all, what technical factors have caused a full-sized low-light unscaled 24mp image to be more noisy than a full-sized unscaled 16mp image?

On a pixel level, the increased visible noise depends in the smaller pixels - at the most extreme, a sensor which captures one photon per photosite will look immensely noise at 1:1, to the point where it's impossible to distinguish the 1:1 crop from random stochastic noise. But at the same time such an image will have the least possible noise when viewed as a whole, since it captures all signal.

This is unintuitive, but worth working through.

Now, if you were asked to reword Professor Bob Atkin's statement (below) to reflect your views about small and big pixels, how would it read? What is wrong with this statement?

"So what you get with sensors is a tradeoff of resolution for noise (and the related dynamic range). Big pixels give you lower resolution, lower noise and higher dynamic range. Small pixels give you higher resolution, higher noise and lower dynamic range.

On a pixel level he's correct. But on the image on the whole there is no such law of nature.


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