From: acl on
On Mar 16, 7:04 am, "Roger N. Clark (change username to rnclark)"
<usern...(a)qwest.net> wrote:

> 2) There is a difference. The signal you record has added
> read noise. A larger pixel collects more photons
> so the signal is larger compared to the read noise.
> Thus you can detect fainter things, or have better high
> ISO performance. If you sum the signal from a smaller
> pixels to equal the area of a larger pixel size,
> you are also adding read noise, so you don't gain
> as much as having the larger pixel with one read noise.
>

Is read noise fixed per pixel, per unit area, or something else?

From: Roger N. Clark (change username to rnclark) on
acl wrote:
> On Mar 16, 7:04 am, "Roger N. Clark (change username to rnclark)"
> <usern...(a)qwest.net> wrote:
>
>> 2) There is a difference. The signal you record has added
>> read noise. A larger pixel collects more photons
>> so the signal is larger compared to the read noise.
>> Thus you can detect fainter things, or have better high
>> ISO performance. If you sum the signal from a smaller
>> pixels to equal the area of a larger pixel size,
>> you are also adding read noise, so you don't gain
>> as much as having the larger pixel with one read noise.
>>
>
> Is read noise fixed per pixel, per unit area, or something else?

Read noise is per pixel. Say you had 2 sensors, one with half
the pixel size, so you needed to add 4 pixels to equal the area
of the larger pixel. Lats say both had great read noise of
4 electrons. The larger pixel gets: X + 4 electrons noise.
The smaller pixel sensor, adding 4 pixels gets:
X + sqrt(4)*4 = X + 8, so the read noise is effectively
doubled.

Read noise for a given sensor is dependent on the design of the sensor
and how the readout is configured. Read ranges from just under 4
to about 30 electrons and is not dependent on pixel size.
For example, see Figure 3 at;
http://www.clarkvision.com/imagedetail/digital.sensor.performance.summary

At low ISO, and low bit count (e.g. 12 bits) and noise in the A/D converter
contributes greater noise than the true read noise from the sensor.

Roger
From: David J. Littleboy on

"acl" <achilleaslazarides(a)yahoo.co.uk> wrote:
> On Mar 16, 6:29 am, "David J. Littleboy" <davi...(a)gol.com> wrote:
>
>> I think you guys are talking past each other here.
>>
>> I think John is arguing that _for a sensor of a given size_, larger
>> pixels
>> aren't any better.
>
> But doesn't this make him a "crop fan" for you?

No. I think what John is saying is orthogonal to sensor size arguments. He's
arguing that for a given sensor size, one wants as many pixels as one can
get. Roger is arguing that for a given number of pixels, one wants the
largest sensor you can get.

I suspect that they're both right.

David J. Littleboy
Tokyo, Japan


From: acl on
On Mar 16, 3:46 pm, "Roger N. Clark (change username to rnclark)"
<usern...(a)qwest.net> wrote:
> acl wrote:
> > On Mar 16, 7:04 am, "Roger N. Clark (change username to rnclark)"
> > <usern...(a)qwest.net> wrote:
>
> >> 2) There is a difference. The signal you record has added
> >> read noise. A larger pixel collects more photons
> >> so the signal is larger compared to the read noise.
> >> Thus you can detect fainter things, or have better high
> >> ISO performance. If you sum the signal from a smaller
> >> pixels to equal the area of a larger pixel size,
> >> you are also adding read noise, so you don't gain
> >> as much as having the larger pixel with one read noise.
>
> > Is read noise fixed per pixel, per unit area, or something else?
>
> Read noise is per pixel. Say you had 2 sensors, one with half
> the pixel size, so you needed to add 4 pixels to equal the area
> of the larger pixel. Lats say both had great read noise of
> 4 electrons. The larger pixel gets: X + 4 electrons noise.
> The smaller pixel sensor, adding 4 pixels gets:
> X + sqrt(4)*4 = X + 8, so the read noise is effectively
> doubled.
>

Yes of course if the noise is independent of pixel size. This is why I
asked.

> Read noise for a given sensor is dependent on the design of the sensor
> and how the readout is configured. Read ranges from just under 4
> to about 30 electrons and is not dependent on pixel size.
> For example, see Figure 3 at;http://www.clarkvision.com/imagedetail/digital.sensor.performance.sum...
>
> At low ISO, and low bit count (e.g. 12 bits) and noise in the A/D converter
> contributes greater noise than the true read noise from the sensor.
>

OK thanks. That explains why it is independent of pixel size. I'd have
thought that, except for noise coming from amplifiers and the readout
circuitry, the rest (things like thermal noise) should scale with the
detector area. But on second thought, this is far more complicated
than the simplistic model I have, there are capacitors, surface states
(for these small sizes) etc.

But personally I hope this kind of noise could go down enough so we
can have high pixel density sensors which will give more flexibility
in trading off noise for resolution. Hopefully with built-in binning
for the raw files too, as always having 60MB raw files seems a bit
wasteful (but the again 300KB for an executable seemed huge to me in
1988).


From: Doug McDonald on
David J. Littleboy wrote:

> No. I think what John is saying is orthogonal to sensor size arguments. He's
> arguing that for a given sensor size, one wants as many pixels as one can
> get. Roger is arguing that for a given number of pixels, one wants the
> largest sensor you can get.
>
> I suspect that they're both right.


It is certainly not true that for a given sensor size, one wants
as many pixels as possible. One does not want pixels so small that
no or very very few lenses can make use of the resulting sensor
resolution, because smaller pixels result in worse very low light
noise. This only occurs when the square root of the number of
photons is smaller than the read noise.

For sensitivity, one does want, for a given number of pixels,
the largest sensor size possible. Of course, to use this one
needs large, fast, expensive lenses ... and the concommittent
loss of depth of field.

Doug McDonald