low light

John Sheehy wrote:
> "Roger N. Clark (change username to rnclark)" <username RemoveThis @qwest.net> wrote
> in news:45FF8497.8020101@qwest.net:
>
>> John Sheehy wrote:
>>
>>> Quantization is just the act of converting analog data to digitized
>>> integers. If there is no added noise in the process, then any analog
>>> range of values equivalent to one ADU will wind up with that single
>>> ADU value. For systems where absolute values matter, this means
>>> errors over any one ADU range, like -0.999 to 0, or -0.5 to +0.499,
>>> or 0 to +0.999; never +/- 1 as Roger suggests in other posts.
>> Gee, some simple research would prove you are wrong.
>
> Gee, maybe you should read what I actually write.

Yeah, some the other way. I always discussed quantization
in terms of the ADC. ADCs are not perfect.

Fine, now I hope we are on the same page.

> If you google my posts in other forums, you will see that I have
> concluded that the flat rate of read noise at all Canon DSLR ISOs
> probably has something to do with the last stages, including the ADC.

Great, we agree, sort of! From the data I see, I conclude
most of the noise at the low ISOs is due to the ADC.

> I
> have not concluded, in along time, however, that it is because of the
> bit depth of the capture.

A better ADC will improve the noise at low ISO. That comes with
more bits (higher bit converters).

> It is easy to quantize data further, and see
> at what point on the quantization curve you are. The fact is, you have
> to quantize ISO 100 by about two bits, and ISO 1600 by about 3 bits,
> before you see more noise, due to the quantization.

This does not make sense.
>
>> Try reading http://en.wikipedia.org/wiki/Analog-to-digital_converter
>> which is a pretty good writ-up.
>> For example, note the statement:
>> "Commercial converters usually have ±0.5 to ±1.5 LSB error in their
>> output." (section on commercial analog-to-digital converters.
>
> If you had paid any attention to what I wrote, you would have seen that I
> wrote "If there is no added noise ...". IOW, I was clearly and
> deliberately taking the mathematical aspect of quantization into
> isolation. I mentioned also in some other spot that it was not 100%
> clear if you were talking about the mathematical act of quantization, or
> the total effect of the ADC, incuding the noise it introduces.

I was only talking about the ADC. That is all that matters in the
quantization step. IT IS ALL ABOUT ADC PERFORMANCE.
12-bit ADCs do not give perfect 12 bits quantization.

> In the past 24 hours, I have had three people on DPReview quote your work
> to me, to prove that the 14 bits in the mkIII will automatically increase
> DR by 2 stops, because current cameras are limited by 12-bit capture.
> had you made it clear that it isn't the bit-depth itself, but the noise
> inherent in real-world ADCs, people might be drawing more accurate
> conclusions.

That is your jump to conclusions. If you read what I actually wrote...
e.g. see the caption to Figure 4 at:
http://www.clarkvision.com/imagedetail/digital.sensor.performance.summary
which I wrote before the 1D mark II was announced:
Figure 4. Dynamic range of sensors. Many sensors are limited to
just under 12 photographic stops by the camera's 12-bit analog-to-digital
(A/D) converter. Look for future DSLRs to use 14 or 16 bit A/Ds.


> There is only going to be 2 more stops of DR if the
> blackframe noise drops to 1.3 14-bit ADUs (0.325 12-bit ADUs). The
> Imaging Resource mkIII had ISO read noises of 4.88 14-bit ADUs and
> greater (I get 7.91 in one file; this may have some kind of electrical
> interference; I have to look closer for patterns).

There won't be 2 stops of improvement with 14-bit ADC if the
Analog Devices ADCs are indicative of the ADC used by Canon.
Canon claims 1 stop of shadow improvement.

>> Let's look at some noise in ADUs from a wide range cameras:
>>
>> Camera Read Noise in ADU (or DN, or LSBs)
>> ISO: 50 100 200 400 800 1600
>> Canon 1DMII 1.2 1.3 1.4 1.7 2.5 4.8
>> Canon 5D 1.8 1.8 1.9 2.1 2.6 7.4
>> Canon 10D 1.4 2.0 3.9 6.4 13.
>
> Those 10D figures are way off. They are 1.9, 2.8, 4.9, 9.0, and 18.0.
> Perhaps your figures were taken from a blackpointed RAW blackframe.

Oh, so you've tested my canon 10D? I didn't see you in my house.
My numbers are correct for my camera.

> The 5D figure is very high for ISO 1600, also. The 5D ISO 1600
> blackframes I have here are all 4.6.

Perhaps there is some variation in cameras, or you are testing
at different temperatures. See reference 13 on my
digital.sensor.performance.summary web page for the 5D data.
>
>> Nikon D50 1.8 4.0
>> Nikon D200 1.3 2.0 3.8 7.4 15.
>
> I don't recall seeing values this low at the low ISOs in the Nikon RAW
> files I had. These are probably taken literally from the RAW blackframe,
> so they are automatically reduced to about 60% of what they'd be if they
> weren't black-clipped, like the Canons.

Well, perhaps you could examine the real data, e.g.:
http://www.clarkvision.com/imagedetail/evaluation-nikon-d200

I don't just do a dark frame measurement; I analyze the
noise and response over the entire range of the sensor and model
the results. See Figure 1 on the above web page. You'll see the
largest deviation from the model is less than 10%, and I have
light levels down to DN 16 (out of 4095). Where is your data
that proves this is wrong?

> You should pay more attention. I never said no noise came from the ADC
> stage or unit; I said the *bit depth* was not the problem.

You've been arguing that a 14-bit ADC would not help the low
ISO performance. Canon and I claim otherwise. Canon has stated
improved shadow noise with their 14-bit converter in the 1DIII.
Current data indicate low ISO cameras (Nikon and Canon) are limited by
12-bit ADCs.

> Let me state my viewpoint with a very clear example; if you quantize a
> 12-bit Canon DSLR ISO 100 to 11 bits, it will lose little DR, much closer
> to 0 stops than 1 stop.
>
> If the 1DmkIII actually had noise of 1.3 14-bit ADUs, and you quantized
> that RAW data to 11 bits, it would still have more DR at the pixel level
> than a 12-bit RAW from existing 12-bit Canons.

This does not make sense.

I think this thread has gone on long enough. Let's simply wait
a few months until 1DIIIs are in the hands of competent testers
and publish real evaluations of read noise and full well
capacities. I predict the read noise in the 1DIII at low iso
will improve by about a factor of 2 over the 1DII simply from
typical ADC specifications.

Oh, and one other prediction: we'll see more images being limited
by photoshop's 15-bit math.

Roger

On Mar 21, 5:27 am, "Roger N. Clark (change username to rnclark)"
<usern....TakeThisOut@qwest.net> wrote:
> Paul Furman wrote:
> > Roger N. Clark (change username to rnclark) wrote:
> >> Let's look at some noise in ADUs from a wide range cameras:
>
> >> Camera Read Noise in ADU (or DN, or LSBs)
> >> ISO: 50 100 200 400 800 1600
> >> Canon 1DMII 1.2 1.3 1.4 1.7 2.5 4.8
> >> Canon 5D 1.8 1.8 1.9 2.1 2.6 7.4
> >> Canon 10D 1.4 2.0 3.9 6.4 13.
> >> Nikon D50 1.8 4.0
> >> Nikon D200 1.3 2.0 3.8 7.4 15.
> >> Canon 20D 2.0 2.2 2.4 3.2 4.5
> >> Canon S60 2.5
> >> Canon S70 2.0 3.4 6.3 17.
>
> > Is there a way to show the 'main' sensor noise in the same units
> > compared to this read noise?
>
> Yes, the ISO 1600 values are pretty close to the true read noise
> of the sensor. So for read noise in ADUs at ISO 100 divide the
> ISO 1600 values by 16. For example, the 1DMII with 4.8 ADUs at
> ISO 1600 should be about 4.8/16 = 0.3 ADU at ISO 100. That is why
> a converter with more bits should improve the low ISO shadow detail.
>
> > I think I understand that an AUD is the
>
> > smallest unit of info that can be read, right?
>
> Yes, ADU. I don't know where this ADU term came from. In the
> terrestrial and planetary sciences, we use DNs, and so do the
> engineers I've worked with on spacecraft sensors.
> DN = data number.
>
> > And these AUDs are
> > essentially rounding errors, not random noise? If so I would expect them
> > to follow a more consistent increas like:
>
> > 1.2 2.4 4.8 9.6 19.2 38.4
>
> The ADUs (DNs) are errors introduced by 1) sensor noise + 2) analog
> gain amplifier noise + 3) A/D converter noise and converter errors.
> It's not a straight line increase because one of those three dominates at
> one end and another dominates at the other end of the ISO.
> #1 and 2 are strongly coupled. 1+2 dominates at the high ISO,
> #3 dominates at the low ISO in the above sensors. We are all


If that's the case, wouldn't on chip binning improve this problem (ie
number 3) at low ISOs? on-chip I mean by reading off 4 (say) pixels at
a time. I know that in CCDs this is not so hard to do for 4 pixels in
a line, but I have no idea if it's equally easy for eg a 2x2 block; I
also don't know if this kind of binning them together introduces other
sources of noise (in which case on-chip binning wouldn't help beyond a
point), or how it works for a CMOS sensor. Do you have any pointers to
more information?