Re: 25 MP sensor of Sony

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.

On the bright/dark range problem, Fuji's "Super
CCD HR" is a brilliant sensor design:

http://www.fujifilm.com/about/technology/super_ccd/index.html

24.8 megapixels measured to 12 good bits would be astonishingly
good. Well, technically good, on those issues. I do realize that
the shortcomings of my own photographs have little to do with
the fidelity of my cameras.

There is no direct relationship between megapixels and
using 12 or 14 bit ADCs, as far as technical image
quality is concerned. The same advantage for a 6 MP
camera is also afforded to a 24 MP camera.

However, *time* is an indirect cost that is significant.
If, for example, all sensor sites were to be processed
by a single channel ADC, obviously a 6 MP camera will
operate 4 times faster than a 24MP camera.

There is little doubt that until the recent camera
models sporting 14 bit ADCs the main reason there were
no 14 bit ADCs was speed, not whether there was or not
any technical quality concerns in the resulting image.
The speed problem has been solved, so we now have 14 bit
ADCs.

--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@xxxxxxxxxx
.

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