Forgive me posting this here - it's not Sony specific, but I am a Sony user and it will get read more here than in some off-topic forum.
But, this is something I only thought about when waking up this morning. It's a universally accepted truth that small sensors gather less light than larger sensors and the former are therefore not as good in low light conditions, producing more noise in high ISO shots.
Everyone knows this, right? I mean it's obvious. The photocells are smaller and they can capture less photos in a given time compared to larger ones. Obviously.
But this is WRONG.
It would be true if the sensor was waved around in the air naked, but in a camera, it is not. The flaw in the paragraph above is that it should read "The photocells are smaller and they can capture less photos in a given time compared to larger ones, given the same level of illumination " And therein lies the flaw.
The amount of light gathered and available for the sensor is not governed by the sensor, it's governed by the lens . If you focus the light from the lens onto a smaller area, the intensity of light increases as the area gets smaller and smaller - ultimately like a magnifying glass in the sun with a spot that is so bright and hot that it burns paper. But all the light is still there, in the tiny spot. That tiny spot is "seeing" all the photons just as the larger magnifying glass lens front is "seeing" them.
Now, clearly if you put a Canon 50mm f/1.4 on the front of an APS-C sensor, in the focal plane, the APS-C sensor takes up less area than an full frame sensor, and so it is true in that case that the smaller sensor gets less of the light.
So my thoughts maybe true, but of only academic interest in a DSLR where things like the distance from the lens rear element to the focal plane are fixed and determined by the system. The smaller sensor sees less of the lens' light and there's not much can be done about it.
But in a proprietary system like a p&s, everything is up for grabs by the designers. It is perfectly possible to have an APS-C sensor gather exactly the same amount of light as a FF one, but focusing all the available light from the lens onto the sensor. The sensor size does not determine how much light the lens captures, the lens does! Putting a bigger sensor behind the glass would not capture any more light if the smaller sensor was already capturing all the light!
So, given the above is true (and thinking about it, clearly it is), I wonder why the small-sensors-gather-less-light myth persists?
Sure, small sensors enable the use of smaller and cheaper lenses, which then gather less light. But that does not need to be the case. Someone could quite easily design a p&s camera witha 1/1.7" sensor that performed just as well as a A900 in terms of light gathering. Practically speaking, you'd have a massive camera though, because the lens would have to be as large as a FF system lens. So the benefit of having a very small sensor is largely eliminated and there would be little point in the design.
And there maybe other practicle issues that influence sensor performance. I am not an expert, but I would not be surprised if heat in the sensor also produces noise and a smaller sensor will be less able to dissipate heat than a larger one.
But consider this. Canon (or whoever) come out with a new p&s and everyone immediately looks to see if the sensor is 1/2.3" or 1/1.7" in size, because they think the larger sensor must gather more light. But this is just plain wrong. Maybe the smaller sensor gathers more? Who knows. It depends on how the lens has been designed, not on the sensor size.
Bizarre, but true!
