What the 'I want more pixels' brigade don't consider!

Started Feb 24, 2013 | Discussions thread
mike703
Veteran MemberPosts: 6,008
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noise
In reply to Simon Devlin, Feb 26, 2013

Simon Devlin wrote:


Is this not the general consensus? Do the manufacturers not have trouble keeping the ISO noise down with higher MP resolutions?

Simon, as someone else has said, this is a very widespread idea - but it is wrong.

As you increase MP count, you decrease pixel size, so the noise *per pixel* goes up because the full well capacity of the pixel is less and variations are proportionately more significant.  But for a given area of the final image, you have more pixels... and the averaging effect decreases the noise.  If you have two sensors of identical technology but different resolution the effect cancels out - if you make the prints the same size which is the only fair comparison as the sensors are the same size.

Suppose you have a 5MP camera with big pixels.  Let's say the full well capacity (bright white at base ISO) is 100,000 electrons.  The noise (error) due to counting the electrons is basically the square root of that, i.e. 316 electrons.  So the noise (random variation from pixel to pixel) is 316 / 100000 which is 0.3%.  Adjacent pixels that should look equally bright actually vary randomly by 0.3% in their brightness - very small.

Now subdivide each pixel into four smaller ones so that you have a 20 MP sensor.  The full-well capacity of each pixel, which is one quarter the original size, is 25,000 electrons.  Noise is square root of that, i.e. 158 electrons.  158 / 25000 is now 0.6% so you have doubled the random variation from one pixel to the next.

BUT... within a given area of the final print, you now have 4x more pixels which vary randomly by +/- 0.6% in brightness.  Some of these variation from one pixel to the next wioll tend to canel count and the noise per unit area decreases as the square root of the number of samples (pixels) in the area.  So 4x more pixels per unit area = half the noise, and you have exactly compensated for the increase in noise per pixel from using smaller pixels.  It's a draw.

In practice, sensors of identical technology but different MP counts are hard to find.  Every time the resolution goes up a bump due to a new generation of technology, the materials in the sensor get better, the electronics get better the microlenses which focus light on the pixels gets better... and so the noise goes down.

The only time when this doesn't hold is when you are taking images in such poor light that you are operating near the read noise limit (the 'noise floor') of the sensor -for example shadows of pictures taken at ISO3200 or ISO6400.  Then, noise is related to number of pixels, but that is normally a tiny effect and does not affect most pictures.

Best wishes

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