From: ejmartin on
On Jul 20, 2:49 am, Bob Newman <bob.csx...(a)gmail.com> wrote:
> On Jul 20, 2:26 am, ejmartin <ejm_60...(a)yahoo.com> wrote:
>
> > On Jul 19, 8:15 pm, John Sheehy <J...(a)no.komm> wrote:
>
> > > "Roger N. Clark (change username to rnclark)" <usern...(a)qwest.net> wrote innews:48815D3C.2030803(a)qwest.net:
>
> > > > John Sheehy wrote:
> > > >>  Then again, I'm
> > > >> the guy who got 100% on all his math and most of his science tests,
> > > >> without studying, so maybe I'm expecting too much.
> > > > So what happened?  I showed you some of your math and conceptual
> > > > errors but you failed to recognize them.
>
> > > I haven't read your posts yet, except for a couple.  It takes a long time
> > > to reply to one of your longer posts, with all the tangents you go on
> > > acting as if I was implying things that I wasn't.
>
> > > The things I've seen of yours in other people's quotes have been off the
> > > mark, and your 4 electron read noise does *NOT* exist in DSLR pixels at
> > > base ISO.  It's a fantasy.  6 or 8 FZ50-like chips fabbed into a larger
> > > wafer with parallel readout is more imminently possible and realistic..
>
> > > --
>
> > John, if you look at any circuit diagram for 4T CMOS active pixel
> > sensors, and compare it to any circuit diagram for a low-noise
> > amplifier, you will see that there is no amplification going on in the
> > pixel.
>
> This isn't exactly the case. That source follower is an amplifier, but
> it's amplifying the current, not the voltage. As it happens, that's
> pretty much equivalent. The charge/voltage gain is supplied by the
> capacitance, as Eric explained. However, since it still only contains
> the photo (and shot noise) electrons, it's ability to drive the
> downstream stages ins not big. The source follower amplifies the
> current, introducing some noise along the way. I think that noise is
> voltage, not charge dependent, but I could be wrong.> Eric Fossum pretty much dismissed your notion that there is
> > special circuitry at the photosite that is ISO dependent, in the
> > thread he started at DPR.  There is no ISO dependent structure at the
>
> There are circuits that can do this, by changing the cell capacitance,
> either by switching in and out capacitors or changing the DC bias on
> the cell, and I was informed by one DPR correspondent that Canon has a
> patent on this. However, I'm pretty sure no current camera does it,
> and Eric seems to confirm.> photosite.  Therefore, in its contribution to read noise, an *upper*
> > bound on the photosite read noise is the lowest read noise (in
> > electron equivalents) as a function of ISO.  This occurs at high ISO.
> > The advantage of FZ50-like pixels at base ISO has little to do with
> > pixel properties.  Since there is nothing going on at the pixel that
> > is ISO dependent, one can infer that the pixel read noise is *at most*
> > the read noise at high ISO (in electron equivalents).  That is 3-4
> > electrons for the best pixels, across the spectrum from small to large
> > pixels and relatively insensitive to pixel pitch.
>
> I don't think we've positively established that the front end read
> noise is truly electron referenced. I would like to see an explanation
> of how it is so. The classical electronics says it isn't, I think we
> have to look at the device physics and quantum effects to understand
> more (i.e. what is the noise contribution of a single electron charge
> on the gate of the source follower)

Yes, but it's neither here nor there. Just treating the sensor as a
black box and running dual amplifications from the output as I have
described in a previous post, one obtains 14 stops of pixel DR. So
unless there is a flaw in that scheme, the result is tantamount to the
assumption that the sensor read noise is 4 electrons at all ISO, since
it results in the same overall system DR as one would get from a
sensor with 14 stops DR and a downstream processing chain that was not
limiting that DR.

But I didn't have to make any assumption that read noise is electron
referred to get the extra two stops of DR, I just used measured
properties from current DSLR's together with the assumption that ISO
amplification is not implemented at the photosite (so that the output
from the sensor could be run through parallel amplification channels
of different gain). If ISO is implemented at the photosite, one could
use the ability to nondestructively read the sensor twice and process
sequentially with the two different gains, with much the same result
(though at a cost in frame rate due to the requirement of sequential
rather than parallel processing).
From: ASAAR on
On Sun, 20 Jul 2008 06:26:42 GMT, John Sheehy wrote:

>> A convenient statement that makes no effort to show and quantify
>> the differences. And we must, of course, assume that FZ50 and 5D
>> circuit differences are negligible, because if not, you'd never have
>> been tempted to use them to make such a meaningless, unfair
>> comparison.
>
> I'm getting really tired of repeating myself.

Which only shows that you haven't been listening.


> Why do you try to create the pretense that I am purposely avoiding a
> responsibility to compare any two cameras that you care to mention?

It seems that you have a rare talent to make your misstatements
less visible by muddying the water through misinterpretation. I
don't want you to compare any two cameras that I or anyone else
mentions. I only gave a couple of examples that disproved your
statement that such similar pairings did not exist. You replied
that the D40 and D40x couldn't be considered because they have
completely different types of circuitry. Please tell us what these
differences are. Will you avoid this question again? Then explain
why the FZ50 and 5D (oops, sorry, 400D) can be fairly compared. I
could be mistaken, but it seems likely that they use types of
circuitry that would be at least as inappropriate for comparison as
the D40 and D40x.


> I can only compare cameras that I have in my posession, with any degree
> of accuracy. Other people's JPEGs are worthless. Other people's RAWs
> are worthless, if lighting and exposure aren't accurately controlled.
> Other people's RAWs are worthless if they use different focal lengths.
>
> Give me any camera you want, and I will compare its pixels and density to
> those of my FZ50 or G9.

I've already given you more than you deserve. It's up to *you* to
select a camera that's appropriate for your comparisons. It can
even be an old, used, very inexpensive camera. If you had a dollar
for each of the hundreds of hours you've been beating this dead
horse here and on DPR, you could have purchased several of these
cameras by now. I wouldn't suggest trying to get the money by
fundraising or begging for dollars because by your insults and tone
(those that disagree with you are stupid), you've alienated what
might have been your potential base.


> Oh, and if certain pairings are so important to you, why can't you
> do them yourself and show us the results?

They're not, see above. It's amazing how you can be wrong so
often yet not recognize that there might be a problem. Especially
since you've already used the term that explains the most likely
reason - cognitive dissonance.

From: Bob Newman on
On 20 Jul, 00:24, John Sheehy <J...(a)no.komm> wrote:
> Bob Newman <bob.csx...(a)gmail.com> wrote innews:dee26fb0-6700-4f10-99a2-1e4eee494cb9(a)b1g2000hsg.googlegroups.com:
>
> >> But then there is no current evidence that there is a dependence of
> >> read noise with pixel size; see Figure 3
> >> at:http://www.clarkvision.com/imagedetail/digital.sensor.performance.s
> >> um...
> > Yes, but as I pointed out above, there's no evidence the other way,
> > either.
>
> There is a loose correlation between read noise relative to maximum signal
> at base ISO and pixel size. The worst DSLRs are worse than average P&S
> cameras, but the best DSLRs have about 1 stop less than the best P&S
> cameras, and we don't know how much of that has to do with the amount of
> money spent on the electronics, as big pixel cameras tend to be more
> expensive cameras, with a more critical consumer base. If more money was
> spent on reading out small pixels, they could conceivably have their read
> noises decreased over what the $150 to $500 camera budgets for compacts
> allow.
>
> --
>
> <>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<>
> John P Sheehy <J...(a)no.komm>
> ><<> <>>< <>>< ><<> <>>< ><<> ><<> <>><
On 20 Jul, 02:26, ejmartin <ejm_60...(a)yahoo.com> wrote:


Ever since I joined this long discussion on DPR those months ago, I've
been puzzled by this read noise thing, and especially by the reference
to electrons. Every way I look at it 'read noise' looks to me to be
voltage referred. If you'll excuse a small treatise on the subject, it
might be interesting. If people can pick holes in it, then I'll learn.
If not, it suggests that you are essentially right.

OK. The canonical analog processing chain, for both CMOS and CCD has a
source follower front end, which current amplifies the voltage on the
pixel, which derives from the cell charge according to the well known
V = Q/C. Being a mosfet, the input current is essentially zero, since
the gate is insulated. This is true unless scales are reduced such
that quantum tunneling of the electrons through the gate becomes
significant. I think, unless shown otherwise, that image sensor
geometries are far away from that (lateral evidence would be that
flash memory uses gate geometries much smaller and manages to retain
gate charges of a few electrons for years). Sometimes, the source
follower will be followed by one or more stages of current or voltage
amplification (the Kodak reference circuits show an emitter follower
to provide further current amplification, followed by whatever voltage
amplification is needed to produce the required full scale input for
the ADC system). Anyway, 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.

Both Roger Clark and Emil Martinec claim that electron referred read
noise is essentially independent of pixel size (and therefore cell
capacitance). I haven't seen any theory to support that, and I don't
think that Roger Clark's measurements, taken as a whole, support it
either. You can find pairs of sensors to compare which suggest any of
three conclusions (small more noise, no dependence, large more noise)
but overall the picture is pretty mixed. To my eyes the biggest
correlations with read noise would be price, manufacturer and date of
introduction. In any case, so far as the downstream electronics are
concerned, the conversion argument above must hold, essentially the
electron referred read noise is scaled by the charge/voltage gain,
which in turn is dictated by cell capacitance. That just leaves the
source follower noise. I cannot find anything to suggest that will be
any different. Again, the noise in this stage is essentially voltage
noise, which must be scaled by the cell capacitance. The noise of a
mosfet is explained here (www.nikhef.nl/~jds/vlsi/noise/sansen.pdf).
Looking at equation 4, it can be seen that this is scale independent -
that is the front end noise will not scale up to compensate for the
capacitance scaling argument I have described.

So, I believe that there is a good argument that read noise, when
electron referred scales down with pixel size. Why isn't this observed
in practice. So much as one can determine a pattern, I believe that
this is dictated by design practice, as much as any underlying
physical reason. Partially, it could be that imager designers are also
looking at things at a pixel level (they certainly would have
designing CCD's for astronomy and similar uses, and standard practices
become embedded. If you photocell design only has 10 stops of dynamic
range, why design for very low read noise, particularly if it makes
things more expensive? There is only a reason to do so if you realise
that a sensor will often be used in a downsampled mode. In essence, if
electron referred read noise is approximately constant it is because
it has been designed to be so. If someone wants to think out of the
box, as you have , there is nothing to stop it scaling down with pixel
area, which will exactly compensate for the aggregation of read noise
from several pixels.

From: Bob Newman on
On 20 Jul, 13:13, ejmartin <ejm_60...(a)yahoo.com> wrote:
> On Jul 20, 2:49 am, Bob Newman <bob.csx...(a)gmail.com> wrote:
>
>
>
> > On Jul 20, 2:26 am, ejmartin <ejm_60...(a)yahoo.com> wrote:
>
> > > On Jul 19, 8:15 pm, John Sheehy <J...(a)no.komm> wrote:
>
> > > > "Roger N. Clark (change username to rnclark)" <usern...(a)qwest.net> wrote innews:48815D3C.2030803(a)qwest.net:
>
> > > > > John Sheehy wrote:
> > > > >> Then again, I'm
> > > > >> the guy who got 100% on all his math and most of his science tests,
> > > > >> without studying, so maybe I'm expecting too much.
> > > > > So what happened? I showed you some of your math and conceptual
> > > > > errors but you failed to recognize them.
>
> > > > I haven't read your posts yet, except for a couple. It takes a long time
> > > > to reply to one of your longer posts, with all the tangents you go on
> > > > acting as if I was implying things that I wasn't.
>
> > > > The things I've seen of yours in other people's quotes have been off the
> > > > mark, and your 4 electron read noise does *NOT* exist in DSLR pixels at
> > > > base ISO. It's a fantasy. 6 or 8 FZ50-like chips fabbed into a larger
> > > > wafer with parallel readout is more imminently possible and realistic.
>
> > > > --
>
> > > John, if you look at any circuit diagram for 4T CMOS active pixel
> > > sensors, and compare it to any circuit diagram for a low-noise
> > > amplifier, you will see that there is no amplification going on in the
> > > pixel.
>
> > This isn't exactly the case. That source follower is an amplifier, but
> > it's amplifying the current, not the voltage. As it happens, that's
> > pretty much equivalent. The charge/voltage gain is supplied by the
> > capacitance, as Eric explained. However, since it still only contains
> > the photo (and shot noise) electrons, it's ability to drive the
> > downstream stages ins not big. The source follower amplifies the
> > current, introducing some noise along the way. I think that noise is
> > voltage, not charge dependent, but I could be wrong.> Eric Fossum pretty much dismissed your notion that there is
> > > special circuitry at the photosite that is ISO dependent, in the
> > > thread he started at DPR. There is no ISO dependent structure at the
>
> > There are circuits that can do this, by changing the cell capacitance,
> > either by switching in and out capacitors or changing the DC bias on
> > the cell, and I was informed by one DPR correspondent that Canon has a
> > patent on this. However, I'm pretty sure no current camera does it,
> > and Eric seems to confirm.> photosite. Therefore, in its contribution to read noise, an *upper*
> > > bound on the photosite read noise is the lowest read noise (in
> > > electron equivalents) as a function of ISO. This occurs at high ISO.
> > > The advantage of FZ50-like pixels at base ISO has little to do with
> > > pixel properties. Since there is nothing going on at the pixel that
> > > is ISO dependent, one can infer that the pixel read noise is *at most*
> > > the read noise at high ISO (in electron equivalents). That is 3-4
> > > electrons for the best pixels, across the spectrum from small to large
> > > pixels and relatively insensitive to pixel pitch.
>
> > I don't think we've positively established that the front end read
> > noise is truly electron referenced. I would like to see an explanation
> > of how it is so. The classical electronics says it isn't, I think we
> > have to look at the device physics and quantum effects to understand
> > more (i.e. what is the noise contribution of a single electron charge
> > on the gate of the source follower)
>
> Yes, but it's neither here nor there. Just treating the sensor as a
> black box and running dual amplifications from the output as I have
> described in a previous post, one obtains 14 stops of pixel DR. So
> unless there is a flaw in that scheme, the result is tantamount to the
> assumption that the sensor read noise is 4 electrons at all ISO, since
> it results in the same overall system DR as one would get from a
> sensor with 14 stops DR and a downstream processing chain that was not
> limiting that DR.
>
> But I didn't have to make any assumption that read noise is electron
> referred to get the extra two stops of DR, I just used measured
> properties from current DSLR's together with the assumption that ISO
> amplification is not implemented at the photosite (so that the output
> from the sensor could be run through parallel amplification channels
> of different gain). If ISO is implemented at the photosite, one could
> use the ability to nondestructively read the sensor twice and process
> sequentially with the two different gains, with much the same result
> (though at a cost in frame rate due to the requirement of sequential
> rather than parallel processing).

We agree that in real, present day cameras ISO doesn't occur in the
photosite. In fact, it doesn't really occur at all, what's happening
is that the capture system is being finagled to best read the bit of
the sensor DR you're actually interested in. As you point out, with a
better capture system, no need for ISO. What I was correcting was your
statement that no amplification occurs in the photocell of an active
pixel sensor. It does (although current rather than voltage
amplification) and its noise is probably the irreducible minimum read
noise. Luckily, if electron referred, it scales down with cell
capacitance, and therefore area (unless you or someone shows
otherwise)
From: John Sheehy on
ejmartin <ejm_60657(a)yahoo.com> wrote in
news:d35ccaac-b701-44df-bd71-f767b7169d79(a)c58g2000hsc.googlegroups.com:

> Yes, but it's neither here nor there. Just treating the sensor as a
> black box and running dual amplifications from the output as I have
> described in a previous post, one obtains 14 stops of pixel DR. So
> unless there is a flaw in that scheme, the result is tantamount to the
> assumption that the sensor read noise is 4 electrons at all ISO, since
> it results in the same overall system DR as one would get from a
> sensor with 14 stops DR and a downstream processing chain that was not
> limiting that DR.

> But I didn't have to make any assumption that read noise is electron
> referred to get the extra two stops of DR, I just used measured
> properties from current DSLR's together with the assumption that ISO
> amplification is not implemented at the photosite (so that the output
> from the sensor could be run through parallel amplification channels
> of different gain). If ISO is implemented at the photosite, one could
> use the ability to nondestructively read the sensor twice and process
> sequentially with the two different gains, with much the same result
> (though at a cost in frame rate due to the requirement of sequential
> rather than parallel processing).

A problem I predict with your system is noise-contouring. I'm not quite
sure I'm ready to look at pulled-up shadows where the read noise at one
level 4.5 stops below saturation is 4 to 7 times as high as a stop below
that level. I'm not a big fan of current HDR; I use it when necessary,
but it is always a hack. If the photosite can be read multiple times
non-destructively, why not just read it multiple times and add the
results together, in an extended-DR mode? You could tell the camera how
many times to read, depending on the amount of time you can allot between
shots, and it would add them in a 16-bit or deeper buffer. That would
give a natural SNR curve with no contouring bands in the noise. Or, if
you can't afford the extra read time and you want different gains, do a
spread of 3 or 4 and blend them into a better curve.

4 stops below saturation at ISO 100 (which would be on the upper side of
your blend, since the top level of the blend must be comfortably below
ISO 1600 saturation) still results in things like chromatic noise in
solid colors in the best DSLRs, unless you have something like a 1Ds3
where it dissolves in the resolution, or print small.

--

<>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<>
John P Sheehy <JPS(a)no.komm>
><<> <>>< <>>< ><<> <>>< ><<> ><<> <>><