Apsc Less is more or is it ?

Started Feb 9, 2013 | Discussions thread
mike703 Veteran Member • Posts: 6,679
Re: The number of pixels and their size does have effect on noise

Mr.NoFlash wrote:

Chris R-UK wrote:

The number of pixels and their size does not have any effect on noise for any given print size. A What does affect noise is the size of the sensor and the sensor and image processing technology.

If the technology is the same a 24MP APS-C sensor will have the same level of noise as a 12MP APS-C sensor.

Not totally true. Lets compare a hypothetical 5 MP sensor and a 20 MP sensor.

Raw noise is photon noise plus read noise.

the photon noise would not change between the 5m sensor and the 20 mp sensor, if the 20 mp sensor would get the same amount of light. while aready that is questionable, problem #2 is a theoretical problem.

the read noise if constant per photodetector. if you have 4 electrons read noise, then on the 5mp sensor you have 4 electrons read noise. on the 20 mp scaled down to 5 mp, you have roughly 4 photodiodes with 4 electrons read noise each of the virtual 5mp photdiodes. if you middle those out, you get a read noise of 8 electrons per 4-photodetector-area. so the read noise is the double amount of you have 4 times the megapixels.

On the other hand, the third question is the technology. That makes sensors which are made 2 years away not comparable currently with the above theories ( maybe that cycle will slow down when some physical limits are hit ).

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Good summary.  If the 5MP and 20MP sensors had the same read noise per photosite, then that part of the noise would increase with number of pixels - at very low light levels where shot noise (photon noise) is small compared to read noise, i.e. you are at the 'noise floor' of the sensor.  At higher light levels the contribution from read noise would be negligible and you'd be in the region where shot noise dominates, and this is independent of number of photosites.

But, as you say - technology improves along with MP counts.  So in the three years or so that it took a 5MP sensor to technologically advance to a 20MP sensor, all the associated electronics have improved and read noise has gone down to compensate for the higher MP count - so a fair comparison based on sensors with identical technolgies but different MP counts probably doesn't exist (as far as I know).

Best wishes

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