From: Roger N. Clark (change username to rnclark) on 20 Jul 2008 17:36
> A bit OT, but as long as you're updating your webpages, one thing I'd
> like to see is the main data table (Table 2) re-sorted by Manufacturer
> (All Canons in one block, all Nikons together, etc) and then within
> that, ordered by camera class and tech generation (eg Canon 10D, 20D,
> 30D, 40D together; 1D, 1D2, 1D3 together, etc). Currently it's a pain
> having to search through the whole list to find a particular camera,
> since even contemporaneous cameras (Nikon D3 and D300) are in
> completely different locations. OK, end of rant ;^)
The table is sorted by pixel pitch as that is what most of the plots
are in. You can always use "find" and search for what you want.
From: Roger N. Clark (change username to rnclark) on 20 Jul 2008 17:41
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
From: Bob Newman on 20 Jul 2008 18:14
On 20 Jul, 22:41, "Roger N. Clark (change username to rnclark)"
> 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. One of
your major criticisms of John's stuff is based on an assumption that
'read noise', referred to photo-electrons, remains approximately
constant. 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'.
From: ejmartin on 20 Jul 2008 18:15
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:
> > 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):
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.
From: ejmartin on 20 Jul 2008 18:45
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, and so at high gain (thus dropping
the effects of Nb) one has
(Cs/Qe)*(Sqrt[kT/Cs] +q Nm +q Nb/Gi)
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?