More megapixels, better photos: Fact or fiction?

Started Feb 7, 2008 | Discussions thread
John Sheehy Forum Pro • Posts: 21,571
Re: More megapixels, better photos: Fact or fiction?

Eric Fossum wrote:

John Sheehy wrote:

The fact is, the more you divide photons into more numerous, smaller
photosites, the less image noise you have to create. The smaller the
total number of photons that need to be read out of a sensor, the
less noise created as measured in photons. The standard deviation of
readout noise in a Canon 5D pixel at ISO 100 with up to 53200 photons
is about 30 photons. The standard deviation of readout noise in a
Panasonic FZ50 at ISO 100, with a maximum of 4800 photons, is about
3.5 photons. A 5D pixel covers about 16x as much area on a sensor as
an FZ50 pixel does. So, 16 FZ50 pixels collect about 4800*16 =
76,800 photons. The read noise for this super pixel is 3.5 photons
times the square root of 16, or 4. 3.5*4 = 14 photons of noise. So,
the 5D pixel has a noise floor a little over a stop higher than the
FZ50 "superpixel", and has slightly more shot noise, with somewhat
less photon collection. This is reality; not the myth going around
that more pixels makes sensors noisier.

No myth John, if I understand what you are saying correctly. Noise
has many components, including photon shot noise that goes like the
sqrt of the number of carriers, and readout noise, which is usually
fixed as a function of signal. We usually care about signal to noise
ratio, SNR. So for shot noise, S divided by sqrt (S) = sqrt(S). So
while the noise is less, when people say "noiseier" they are almost
always talking about SNR, which degrades with smaller pixel sizes for
the most part.

Did I understand you correctly?

The SNR as you describe it is not relevant to the issue of pixel density. the integrity of an image is not dependent directly upon the integrity of each pixel, and the SNR that you refer to is the SNR of a pixel. It is meaningless for the purposes of imaging taken alone, except that it is a single factor in image noise.

Imagine that it was your job to record the pattern of raindrops that fell through a tree, and dripped off of its branches and leaves, and you were given 16 square bins to put in a 4x4 pattern under a tree. However, you thought that you'd be clever, and replaced each bin with 4 smaller ones. Upon collecting the data (measuring the water in each bin), you gave the person in charge the data for 64 bins, instead of the 16 you were instructed to measure. What is the problem? How is this any different than counting photons?
--
John

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