Re: 25 MP sensor of Sony

Floyd L. Davidson <floyd@xxxxxxxxxx> wrote:
Bryan Olson <fakeaddress@xxxxxxxxxxx> wrote:
frederick wrote:

The argument that 12 bits is enough challenges the assumption:
"presuming that the sensor is capable of recording that depth
of data." Consider the signal that will saturate a sensor cell,
so that more light will not increase the change. If a signal
1/4096 as large is significantly less than the absolute noise,
then 12 bits can cover the sensor's dynamic range.

That is true, but can be misunderstood too. What does
"absolute noise" mean? You go on to discuss "dominant
noise", which is a different beast.

Roger M. Clark has argued that the dominant noise is "shot
noise," which is in the light itself; even a sensor that counts
photons perfectly would have this shot noise.

Shot noise is dominant above some given level of
illumination. It is not dominant at lower levels.
Therefore perhaps the most obvious problem with the rest
of this discussion is the assumption that shot noise in
some way relates to the "absolute noise" mentioned
above. It does not.

The "absolute noise" reference is to the noise when
there is no light (i.e., no signal). That is read
noise. It is random.

As sensor cell
size gets smaller, so does the limiting signal-to-noise ratio.
The signal is proportional to the sensor-cell size, while the
noise is proportional to the square root of the signal.
(Technically, photon arrival is well-modeled as a "Poisson
process".)

The noise in the high signal areas is photo noise. The
noise in low signal areas is not. That is where dynamic
range is important.

Thus the smaller the cell size, the fewer bits needed to cover
its dynamic range. I have not examined the numbers as well as
has Roger, but putting out the signal from a 5.94?m sensor cell
beyond 12 bits of precision is a bit like saying the Earth will
be four and half billion years old next Tuesday.

Capturing bright highlights and dark shadows in the same shot
is a different issue from the bits needed to cover a cell's
dynamic range.

If the bits needed to cover the cell's dynamic range are
not there, whatever range of bright to dark the cell can
capture is *lost*. It *is* exactly the issue. One cell
may be at it's brightest, and another cell at it's
darkest... they *both* must be encoded in the digital
signal.

They can be encoded with any number of bits from 1 upwards. You don't
lose dynamic range with fewer bits, you lose fineness of gradation
steps. With one bit all you get is white and black. With four bits you
get two intermediate shades of grey. With 12 bits you get 4000 shades
of grey. I'm pretty sure I can't distinguish 4,000 different shades of
grey, so I'm sure 12 bits is more than enough for the dynamic range of
my eyes, other things being equal. There's no reason why a 10bit ADC
couldn;t cover 10 times the dynamic range of a 15 bit ADC. However,
it's often technologically convenient to use more bits than necessary
for a variety of reasons, and those reasons change with the
technology.

Given Sony's different implementation of the ADCs their 12 bits might
in fact offer less noise and more dynamic range than the usual 14
bits, just as X's 12MP camera might offer higher resolution images
than Y's 13MP camera. It's not just the numbers.

--
Chris Malcolm cam@xxxxxxxxxxxxxxxxxxxx DoD #205
IPAB, Informatics, JCMB, King's Buildings, Edinburgh, EH9 3JZ, UK
[http://www.dai.ed.ac.uk/homes/cam/]

.

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