Question regarding resolution and noise

[J. McCabe]

It has been noted before that the smaller the pixel, the less light it gathers, and the larger the S/N ratio.

My question is whether this is true per pixel, or per unit of area.

To clarify, compare two sensors of same technology, sensor A having X pixels, and sensor B 4X pixels. Four of B's pixel would cover the same area as one of A's pixel, and would thus collect the same amount of light. Would having four readout circuits necessarily increase the read-out noise, leading to smaller S/N ratio per unit of area ?

 

[Loswr]

It has been noted before that the smaller the pixel, the less light it gathers, and the larger the S/N ratio.

My question is whether this is true per pixel, or per unit of area.

To clarify, compare two sensors of same technology, sensor A having X pixels, and sensor B 4X pixels. Four of B's pixel would cover the same area as one of A's pixel, and would thus collect the same amount of light. Would having four readout circuits necessarily increase the read-out noise, leading to smaller S/N ratio per unit of area ?

Slightly, but probably the difference would get lost in other factors - sensor A and sensor B might have the 'same technology' but they would likely be supported by different image processors (like Canon's Digic or the equivalent), and running different firmware. That's why even cameras with the same sensor (550D, 600D, 60D, 7D) have slightly different IQ and noise characteristics. Those differences would outweigh the slight loss of S/N with the higher density pixel array.

Of course, the above assumes sensor A and sensor B are the same total area - noise is primarily driven by total light gathered, if sensor B was larger than sensor A, even with 4x the pixels per unit area, sensor B would have far less noise nativley from the sensor