From: Roger N. Clark (change username to rnclark) on 20 Jul 2008 19:32
Bob Newman wrote:
> On 20 Jul, 22:41, "Roger N. Clark (change username to rnclark)"
> <usern...(a)qwest.net> wrote:
>> Bob Newman wrote:
>>> Yes, this is confusing, but working in one unit only also causes
>>> confusion. Particularly concerning 'read noise' in electrons, when
>>> it's a voltage referred noise.
>> Scientifically, it is better in my opinion (as a scientist) to
>> work in units that relate to the scene. That could be watts/sq meter,
>> photons, or in this case electrons which can be directly
>> correlated to the photons captured. In my scientific work,
>> we always work in units related to the subject, not voltage in
>> the instrument.
> That's fair enough. The problem is, when it gets in the way of people
> conceptualising what's actually happening. In this case, the photo-
> electron referred noise figure is distinctly unhelpful in working
> one's way through the actual noise sources, and the variously
> amplified versions of them which appear in the final signal.
But we consumers can not measure these engineering parameters unless
we disassemble the cameras. We can only measure what is produced
by the camera output for a given known input. Having zero
initial information it is pretty impressive that we can derive
the photon count in the first place, as long as we can get access
to raw data.
> One of
> your major criticisms of John's stuff is based on an assumption that
> 'read noise', referred to photo-electrons, remains approximately
I never said read noise is constant, at least between sensors.
What I said was there was NO correlation of read noise with pixel size.
Read noise is independent of ISO because ISO is a post sensor read gain.
Read noise varies a lot between different sensors because of different
methods for reading the signal and suppressing noise.
> I cannot see why this should be so, since the translation
> from what it is (a noise current in the amplifier stage) to apparent
> photo-electrons must be due to the cell capacitance, which is at least
> loosely related to cell area. The real truth is that a lot of the
> assumptions are based on observations of sensors which are not things
> that occur naturally in inevitable configurations. What you are
> observing is the result of conscious design choices, to use them as
> evidence of an inevitable trend is hardly 'scientific'.
You might have a look at:
The pixel and sensor size issue is well known in systems design
for scientific instruments, on aircraft, telescopes, and spacecraft.
See, for example, http://en.wikipedia.org/wiki/Etendue
There are scientific reasons for observed trends.
From: David J Taylor on 21 Jul 2008 02:30
Ray Fischer wrote:
> David J Taylor
> <david-taylor(a)blueyonder.neither-this-bit.nor-this-bit.co.uk> wrote:
>> Ray Fischer wrote:
>>> David J Taylor
>>> <david-taylor(a)blueyonder.neither-this-bit.nor-this-bit.co.uk> wrote:
>>>> Ray Fischer wrote:
>>>>> Who cares? People don't care about "pixel densities". People
>>>>> care abut noise per pixel, not noise per sensor area.
>>>> So why buy a 12MP APS-C DSLR over a 6MP APS-C DSLR if people don't
>>>> care about pixel densities?
>> Which suggests that you /do/ care about the pixel density....
> No, it says that they care about the number of pixels. Notice that
> didn't ask about pixel density? You asked about number of pixels?
> That should have been a clue.
You said: "People care abut noise per pixel, not noise per sensor area".
Now you say "they care about the number of pixels". Which is it to be? I
was careful to specify the same size sensor.
Although some would choose 6MP, I bet a majority would choose 12MP, which
suggests to me that either marketing has succeeded, or that they really do
prefer resolution over noise.
From: Bob Newman on 21 Jul 2008 04:39
On 20 Jul, 23:15, ejmartin <ejm_60...(a)yahoo.com> wrote:
> On Jul 20, 3:37 pm, Bob Newman <bob.csx...(a)gmail.com> wrote:
> > On 20 Jul, 18:10, ejmartin <ejm_60...(a)yahoo.com> wrote:
> > > On Jul 20, 11:30 am, Bob Newman <bob.csx...(a)gmail.com> wrote:
> > > > I think currently realised is the issue at the moment, both for sensor
> > > > technology and signal processing chain. In the best of all possible
> > > > worlds many things might be possible on both fronts. EF certainly
> > > > seems to have something up his sleeve, and from his slide show, it
> > > > seems to include really tiny pixels and really big DR. In the limit,
> > > > when you get to a true digital sensor, when each pixel has a FWC of
> > > > 1e, read noise ceases to be an issue. Near that limit, if each pixel
> > > > has a FWC of 2e, it's not much of an issue. This is another thing that
> > > > makes me think that you and Roger are not right, fundamentally, on
> > > > this. Somewhere between here and there, there would need to be a
> > > > turning point when the read noise issue stopped getting worse and
> > > > started getting better. In fact, I'm beginning to think I could mount
> > > > an inductive proof that you are wrong.> There may be in some hoped-for future a means of lowering the small
> > > > > pixel read noise to about 1 electron (input referred), which is not
> > > > > simultaneously available for bigger pixels; perhaps the reason will be
> > > > > the sort of capacitance arguments you have put forth. At that point,
> > > > > small pixel DR on a per area basis will equal that of the 1D3's fully
> > > > > realized sensor DR, and small pixels will be competitive on SNR and
> > > > > DR. But there is no such pixel like that among current examples.
> > > > John and I would say, because no-one has bothered to develop it,
> > > > because it lies so far off the accepted orthodoxy of camera design. I
> > > > think that's the way Eric's going, though.
> > > I just remembered, while we are waiting for production small-pixel
> > > CMOS sensors from Canon, there is one further data point: that 52MP,
> > > APS-H sized sensor that they made a prototype of:
> > >http://www.imagesensors.org/Past%20Workshops/2007%20Workshop/2007%20P....
> > > 3.3µ pixels, with 5.5 electrons of read noise.
> > With a column gain of 3, look at figure 5. Who knows how that compares
> > with the figures obtained using the various amateur testing methods.> Still not getting
> > > smaller in proportion to pixel pitch (in fact, a bit worse; granted,
> > > it's preproduction, but if it were easy to beat down the read noise by
> > > making the pixels smaller, shouldn't they have been able to at least
> > > match the performance of pixels with 4 or more times the area?).
> > Who knows also what the design goals were? There are reasons to keep
> > the cell capacitance high, which will increase electron referred read
> > noise but will also increase FWC and therefore DR. Read noise is a
> > parameter the designer can trade off against other things, unless you
> > know what those tradeoffs were, you can't draw any hard conclusions.
> One thing we can compare is DR per area; that seems to me pretty
> unambiguous (or is that mistaken?). Read noise per area scales as
> read noise per pixel divided by the sqrt of the number of pixels per
> area; FWC per area is the FWC per pixel divided by the number of
> pixels per area; thus the DR per area is some fixed number times FWC
> per pixel/(read noise per pixel * pixel spacing):
No, I don't think it's ambiguous, and I think it is a good figure of
> 1D3: 71000 electrons FWC/(4.0 electrons read noise * 7.2µ pixel
> spacing) ~ 2470
> 40D: 40000 electrons FWC/(4.3 electrons read noise * 5.7µ pixel
> spacing) ~ 1630
> 52MP prototype: 24000 electrons FWC/(5.5 electrons read noise * 3.2µ
> pixel spacing) ~ 1370
> The trend to me seems rather clear.
Yes, except that that sort of data analysis wouldn't pass muster in
any serious context. We have three samples which appear to show a
trend. If we accept that there is a trend (and three samples is a
rather small number on which to base it!) we still don't know why
there's a trend. As I suggested to Roger Clark, these are designed
artifacts, we do not know what the designer is aiming for. I'd hazard
a guess that designers of high megapixel DSLR's are aiming at maximum
IQ at low ISO's, in which case they'll put the capacitance of the
sensel as high as feasible within the space constraints of the pixel,
since increased FWC gives increased pixel level DR. This will
adversely affect read noise/ area.
> - Hide quoted text -
> - Show quoted text -
From: Bob Newman on 21 Jul 2008 04:51
On 20 Jul, 23:45, ejmartin <ejm_60...(a)yahoo.com> wrote:
> On Jul 20, 9:37 am, Bob Newman <bob.csx...(a)gmail.com> wrote:
> > let's call the noise produced by the source
> > follower and any subsequent fixed gain amplifier the 'front end read
> > noise', Nf
> > The source follower is followed by one or more stages of voltage
> > amplification and one or more stages of programmable gain
> > amplification. Let's call these 'middle read noise, Nm'.
> > Finally, we have the ADC system, which generally consists of a sample-
> > and-hold (for correlated double sampling) an amplifier and the ADC.
> > let's call this the 'back end read noise', Nb.
> > Assuming that all the three noises are produced by a single stage of
> > amplification, without overall feedback (which isn't always the case)
> > and that all the voltage gain is in the ISO gain stage (also a
> > simplification, but not one which affects the following argument) then
> > the 'read noise' recorded by the ADC is Gi*(Nf +q Nm) +q Nb. (where +q
> > is shorthand for adding in quadrature) This assumes that the variable
> > gain amplifier is a well designed feedback controlled amplifier, and
> > its noise is somewhat independent of gain.
> > Firstly, why does 'read noise' reduce with ISO? Of course, in reality
> > it doesn't, but it appears so if we relate it to the photoelectrons in
> > the sensel. To reference the read noise to electrons, we need to take
> > into account the charge/voltage gain, which is given by the sensel
> > capacitance (Cs), the charge of an electron (Qe) and the voltage gain
> > of the chain, so the electron referred noise is (Cs/Gi*Qe)*(Gi*(Nf +q
> > Nm) + Nb). If we re-arrange that we get (Cs/Qe)*(Nf +q Nm +q Nb/Gi).
> > So we can see, if we want to 'electron refer' the read noise, we
> > divide the back end noise by the gain, which is higher at high ISO's.
> > If that gain is high enough, the back end noise becomes insignificant.
> > Back to pixel size and read noise. The sensor measurers have
> > established a standard practice of measuring read noise in electron
> > equivalents, as though they were noise in the pixel itself. This means
> > 'passing' the noise 'backwards' through the charge/voltage converter,
> > which is the cell capacitance. This must mean that the electron
> > referred read noise depends on the cell capacitance, which will mean
> > it tends to reduce as pixel sizes reduce. If this is the case, it
> > removes the argument that small pixels contribute more read noise per
> > unit area.
> Carrying this analysis a step further, can we assume that Nf is
> thermal noise? Then <V^2>=kT/C,
Yes, mainly thermal noise, but noise in the mosfet channel, not the
cell capacitance. Obviously there is some of that too, but I think
it's usually agreed not to be significant. And it's not rread noise.
> and so at high gain (thus dropping
> the effects of Nb) one has
> (Cs/Qe)*(Sqrt[kT/Cs] +q Nm +q Nb/Gi)
No, the first noise term is given by mosfet noise equations, as the
source I gave. As well as the channel mobility (which is adjustable by
doping) this turns out to be proportional to the width/length ratio
(long thin transistors are quieter than short fat ones). This is in
the limit a constraint for small pixels, since the process geometry
determines just how long and thin you can make your fet, and of course
the longer you make it, the bigger the gate capacitance and the less
the cell capacitance scaling effect.
Got to go to a meeting now! Follow up the rest later.
> Cs should be proportional to the collection area, as this gets
> asymptotically small the input-referred noise should scale according
> to this formula as the sqrt of the collection area, ie with the linear
> size of the pixel. Actually it would decrease somewhat faster than
> that, for a given level of technology the size of the support
> electronics is fixed and the collection area will decrease *faster*
> than linearly with the pixel spacing. We can make the input referred
> read noise as small as we want if we let the photosensitive area go to
> If the collection area is Ac and the support electronics occupies Ae,
> and the pixel spacing is d, one has d^2=Ac+Ae. The FWC goes as Ac,
> the read noise as sqrt[Ac], and the DR per area is (see above post)
> DR/area ~ const * Ac/(sqrt[Ac] * d) ~ const * sqrt[1-(Ae/d^2)]
> So with these assumptions -- fixed area requirements for support
> electronics -- DR per area goes down as the pixels are shrunk. One
> can only decrease pixel spacing and maintain DR per area if the
> support electronics shrinks in proportion to the pixel size, which
> makes a lot of sense.
> One might also worry that there are input referred noises that might
> not scale. Can we be sure that there are no constant sources of input-
> referred noise, for instance noise in the 4T arrangement that reads
> out the photoelectron count?- Hide quoted text -
> - Show quoted text -
From: Steve on 21 Jul 2008 08:42
On Mon, 21 Jul 2008 06:30:04 GMT, "David J Taylor"
>Ray Fischer wrote:
>> David J Taylor
>> <david-taylor(a)blueyonder.neither-this-bit.nor-this-bit.co.uk> wrote:
>>> Ray Fischer wrote:
>>>> David J Taylor
>>>> <david-taylor(a)blueyonder.neither-this-bit.nor-this-bit.co.uk> wrote:
>>>>> Ray Fischer wrote:
>>>>>> Who cares? People don't care about "pixel densities". People
>>>>>> care abut noise per pixel, not noise per sensor area.
>>>>> So why buy a 12MP APS-C DSLR over a 6MP APS-C DSLR if people don't
>>>>> care about pixel densities?
>>> Which suggests that you /do/ care about the pixel density....
>> No, it says that they care about the number of pixels. Notice that
>> didn't ask about pixel density? You asked about number of pixels?
>> That should have been a clue.
>You said: "People care abut noise per pixel, not noise per sensor area".
>Now you say "they care about the number of pixels". Which is it to be? I
>was careful to specify the same size sensor.
>Although some would choose 6MP, I bet a majority would choose 12MP, which
>suggests to me that either marketing has succeeded, or that they really do
>prefer resolution over noise.
Or, that for an APS-C sensor size, you're in the area of the S/N vs.
Pixel Density curve where it's still relatively flat. So you don't
give up all that much S/N when moving from 6 to 12MP for an APS-C DSLR
like you would for a 1/1.8 pocket camera, which has about 10 times
less sensor area.