Read noise: get it 30x down?

Ilya Zakharevich wrote:
> [A complimentary Cc of this posting was sent to
> Scott W
> <biphoto@m>], who wrote in article <46fefd97$0$26341$4c368faf@roadrunner.com>:
>> As Roger Clark has pointed out the read noise is pretty low in most
>> cameras and quantizing noise is going to dominate, in which case there
>> would be little benefit in reducing it by a factor of 10.
>
> ??? Roger has his blind spots, but on this topic he is quite kosher.
> He would not say anything as wrong as this.
>
> Just look at his tables: quantization dominates the low-ISO region.
> Read noise dominates high-ISO region.
>
> And adding another bit or two to quantizer is not a big deal.
> Decreasing readout noise may be quite tricky...
>
>> The other part is that part of the noise will be how evenly the quantum
>> well are filled at the beginning of an exposure, if there in noise here
>> multiple reads will not help. In other words can you will a 50,000
>> electron well within an electron or 2 from cell to cell?
>
> Readout noise of 4 electrons matters only on the individual cells
> where the exposure is below about 25 electrons. So full well of 50Ke
> is not relevant.
>

In a CCD the wells start full and discharge, so a 25 electron exposure
with a 50,000 electron well will take you from 50,000 to 49,975. In
which case you better be very careful to get 50,000 in your well be for
exposure and not say 50,010. This error in filling the wells will show
up as read noise, and doing multiple reads on a cell will not lower the
this component of noise.

Scott

Doug McDonald <NOmcdonald@SPscs.AMuiuc.edu> wrote in news:fdn4p3$9m6$1
@news.ks.uiuc.edu:

> On the other hand, if you get the thermal noise down enough,
> and the actual quantization noise low enough, you can, at least at low
> light levels, figure out the number of electrons exactly. This
> gets you to the ultimate "perfect" situation.
>
> I have no idea whether this is feasible on a CCD or CMOS sensor
> running at camera speeds.

Then the problem becomes, "how do you do ISO 100 on the same camera?".

Maybe we'll see dedicated low-light bodies in the future?

I believe that shot noise, in isolation, without any read noise, is much
less distracting than (current levels of) read noise, in very low photon
captures. I've simulated pure shot noise from well-exposed ISO 100 images,
and the shot noise looks quite nice compared to the garbage we call read
noise and total blackframe noise. Read noises always have a chance to be
patterned; shot noise can not be, not with any reasonable chance. Shot
noise looks like a texture in the subject; read noise looks like a film of
streaks, smears and fireworks super-imposed upon the image.

--

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John P Sheehy <JPS@no.komm>
><<> <>>< <>>< ><<> <>>< ><<> ><<> <>><

On Sep 30, 8:12 pm, John Sheehy <J...@no.komm> wrote:
> Read noises always have a chance to be
> patterned; shot noise can not be, not with any reasonable chance. Shot
> noise looks like a texture in the subject; read noise looks like a film of
> streaks, smears and fireworks super-imposed upon the image.

Is the patterned read noise consistent from frame to frame? That is,
if I take two blackframe images and subtract them, does the patterned
noise cancel, or is it simply patterned, with the precise pattern
varying from image to image (so that the subtracted images would have
sqrt(2) as much pattern noise)?

ejmartin <ejm_60657m> wrote in
news:1191215149.253354.299560@22g2000hsm. :

> Is the patterned read noise consistent from frame to frame? That is,
> if I take two blackframe images and subtract them, does the patterned
> noise cancel, or is it simply patterned, with the precise pattern
> varying from image to image (so that the subtracted images would have
> sqrt(2) as much pattern noise)?

There are some cases, and it is certainly true with long exposures, but
there are non-single pixel noises that do not repeat in successive frames
in many cameras. Most often, it is an offset in the blackpoint that occurs
for only a fraction of a pixel line. Offsets that affect the entire line
could be removed with data obtained from unexposed pixel borders (Canon
RAWs maintain black pixels in the RAW file from the top and left edges of
the image), but artifacts that occur mid-line can not be positively
identified.




--

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John P Sheehy <JPS@no.komm>
><<> <>>< <>>< ><<> <>>< ><<> ><<> <>><

On Oct 3, 8:26 am, "Roger N. Clark (change username to rnclark)"
<usern...@qwest.net> wrote:
> Floyd L. Davidson wrote:


> > Which means of course that the ADC is obviously pretty
> > damned good, and "good enough" at the least.
>
> No, they are not. For example, the sensor read noise of
> the 1D Mark II and 5D are about 4 electrons or a little less.
> Full well capacities are about 80,000 electrons for a
> sensor dynamic range of 80000/4 = 20,000 (14.3 stops)
> (or a little more). That's 86 dB!

I've been meaning to mention this for some time but kept forgetting:
Your results for the 4 e^- read noise are at ISO 800. At ISO 100, how
do we know that the linearity of the amplifiers wouldn't be
compromised? They'd need to be able to output a voltage 8 times
higher at ISO 100 while still responding linearly if we're going to be
able to use the full DR of the sensor.... Any idea if this is a
limiting factor or not?

"Roger N. Clark (change username to rnclark)" <username@qwest.net> wrote in
news:470390C6.1080509@qwest.net:

> The sensor data clearly shows the sensors are capable of
> very high dynamic ranges, >20,000.

The wells are capable of higher DR, but the readout circuitry can not
deliver. You seem to believe that Canon DSLRs have a single on-photosite
amplification, and that differences in blackframe noise at different ISOs
for short exposures are due to the ADC noise. I really don't see any
evidence to support that. I see it in many other brands' DSLRs; I do not
see it in Canon. The evidence suggests that *at* the photosite, there are
different read noises in electrons at different ISOs:

1) If you try to account for blackframe noise at various ISOs by the square
root of the sum of a fixed photosite read noise squared and a scaled ADC
noise squared (both in electrons), you can find values that satisfy both
ISO 100 and 1600, but they won't satisfy ISO 400 at all.

2) On the Canon 5D and 1D* cameras, ISOs 100, 125, 160 have almost exactly
the same blackframe noise in electrons. ISOs 200, 250, and 320 have the
same blackframe noise in electrons, but less than the former group, etc.
There are no gaps or spikes in the histograms of the extra ISOs, compared
to the main ones, so these ISO are clearly done by a second level of
amplification, which suggests that each ISO in the groups of three have a
unique amplification at the photosites, because if all the ISOs were
achieved by second-stage amplification, there would be a smooth
relationship between blackframe noise and ISO.


IMO, the real problem is that the readout circuitry just isn't up to par
for reading large photon counts, and that is one reason why I think smaller
pixels are generally better for image-level blackframe noise.

--

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John P Sheehy <JPS@no.komm>
><<> <>>< <>>< ><<> <>>< ><<> ><<> <>><

On Oct 3, 8:02 am, "Roger N. Clark (change username to rnclark)"
<usern...@qwest.net> wrote:
> Floyd L. Davidson wrote:
> > "Roger N. Clark (change username to rnclark)" <usern...@qwest.net> wrote:
> >> Floyd L. Davidson wrote:
> >>> Which means of course that the ADC is obviously pretty
> >>> damned good, and "good enough" at the least.
> >> No, they are not. For example, the sensor read noise of
> >> the 1D Mark II and 5D are about 4 electrons or a little less.
> >> Full well capacities are about 80,000 electrons for a
> >> sensor dynamic range of 80000/4 = 20,000 (14.3 stops)
> >> (or a little more). That's 86 dB!
>


Roger, I've never understood this aspect of your reasoning. What
allows you to divide the clipping point at one ISO by the read noise
at another ISO to get the dynamic range? I would be taller if I
measured the height of my head while standing on a ladder and the
depth of my feet standing on the ground. Don't we have to fix the
conditions of the measurement when we are making it?

On Oct 4, 6:34 am, "Roger N. Clark (change username to rnclark)"
<usern...@qwest.net> wrote:
> acl wrote:
> > On Oct 3, 8:26 am, "Roger N. Clark (change username to rnclark)"
> > <usern...@qwest.net> wrote:
> >> Floyd L. Davidson wrote:
>
> >>> Which means of course that the ADC is obviously pretty
> >>> damned good, and "good enough" at the least.
> >> No, they are not. For example, the sensor read noise of
> >> the 1D Mark II and 5D are about 4 electrons or a little less.
> >> Full well capacities are about 80,000 electrons for a
> >> sensor dynamic range of 80000/4 = 20,000 (14.3 stops)
> >> (or a little more). That's 86 dB!
>
> > I've been meaning to mention this for some time but kept forgetting:
> > Your results for the 4 e^- read noise are at ISO 800. At ISO 100, how
> > do we know that the linearity of the amplifiers wouldn't be
> > compromised? They'd need to be able to output a voltage 8 times
> > higher at ISO 100 while still responding linearly if we're going to be
> > able to use the full DR of the sensor.... Any idea if this is a
> > limiting factor or not?
>
> It does not appear to be a factor in the scientific sensors,
> but they are running at a lower speed.

OK thanks.

> Also, there is no need to
> output voltages 8 times higher; instead, noise needs to go down 8x.
>

Temporary mental shutdown, sorry.