From: Roger N. Clark (change username to rnclark) on
acl wrote:

> But I find it very hard to believe that this limit is at 6 microns, or
> that the performance of the canon cameras in terms of read noise
> cannot be duplicated and improved.

6-micron limit? Who said 6-micron limit? I haven't said anything
about a 6 micron limit. I've only argued that the premise
of binning smaller pixels can equal the performance of larger pixels.
That goes for 8 versus 16-micron pixels or 2 versus 6.
I also said given the choice of a 200+ megapixel camera with
2-micron pixels or a 24 mpixel camera with 6 micron pixels,
I would choose the 200+ camera for static shots and the
24 mpixel camera for fast action and low light work.

From: John Sheehy on
Doug McDonald <mcdonald(a)> wrote in

> John Sheehy wrote:
>> What are you comparing it against? Let's say you have 9 tiny pixels
>> with a signal of 5 electrons, and a read noise of 2.7 electrons. 9
>> of those, binned together, have a signal of 45 electrons, and a read
>> noise of 8.1 electrons. With hardware binning like Dalsa uses, that
>> might mean 2.7 electrons (or a tad more) even for the binned
>> superpixel.

> Hardware binning, that is, adding the charge before the amplifiers,
> does indeed work fine, except you lose the resolution!

> As I said in another post, all else (i.e. fill factor) being
> equal, smaller pixels do result in a smaller read noise due
> to smaller capacitance. The downside of course is lower
> dynamic range.

Why? DR depends on noise. Less read noise means higher low-standard (1:1
SNR) DR, and the same number of photons means the same shot noise, and
therefore the same high-standard (10:1 SNR) DR.

This all works with or without binning. Don't forget, light is really
falling everywhere with extreme resolution, and extreme shot noise. Just
looking at it that way doesn't make it any noisier than it is.


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John P Sheehy <JPS(a)no.komm>
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From: John Sheehy on
"Roger N. Clark (change username to rnclark)" <username(a)> wrote
in news:45FC7FEE.7010800(a)

> Your number for the 20D is incorrect. What you refer to as read noise
> is not true sensor read noise, but the limitation of a 12-bit A/D
> converter.

I didn't say that it was "true sensor read noise". I said that it was
"read noise" and sometimes I qualify it by saying "blackframe read

Regardless of the source, it is making the shadows of the RAW files much
less useful than what the sensor wells themselves record.

> The 20D has a full signal well depth of 51,000 electrons.

No, it has about 44,000. It has about 26,500 at RAW saturation at ISO
200, but ISO 100 has less headroom, and the camera stretches the upper
highlights to reach 4095 ADUs, like the other ISOs on the camera.

Here is the same scene, with twice the exposure time on the left, ISO 100
on the left, and ISO 200 on the right:

The ISO 100 comes to clipping faster in the gradient, even though all the
darker tones are exactly the same.

> So: 51000/4095 = 12.5 electrons per A/D bit.

The 20D has 3967 meaningful ADUs; 0 through 127 are only for negative

51000/3967 = 12.85 electrons per ADU. Read noise is 2.07 ADU at ISO 100.
Read noise is then 12.85*2.07 = 26.61 electrons.

> Add +/- 1 bit noise
> on each A/D reading,

+/- 0.5 ADU, with a global offset of +/- 0.5 ADU, which should be
accounted for in blackpointing the RAW data, and the former adds like any
other noise; the square root of the sum of the squares, so you don't add
it linearly. It sometimes goes positive when the analog noise goes
negative, and visa-versa.

> and noise should be about 1.4 bits, or
> 1.4 * 12.5 = 17.5

What kind of math is that? You're multiplying a term called "bits" which
should be a logarithm, I think, by a linear ratio. Also, where did the
1.4 come from? The noise in an ISO 100 blackframe from a 20D is about
2.07 ADU. 2.07 * 12.5 = 25.87. 2.07 * 12.85 = 26.61. Neither is close
to 17.5.

> electrons not including actual sensor read
> noise. DSLRs are so good at low ISOs, they are limited at the
> low end by A/D converter electronics, not sensor read noise.

Whatever it is, it is a problem. 2.07 ADU of noise is not caused by bit
depth. It is analog noise, regardless of where it happens in the signal

> Small pixel P&S cameras get so few photons, that the entire range
> is adequately characterized by 12-bit converters.

No argument there, for high ISOs. The FZ50 has 4800 photons per pixel,
and 3982 (IIRC) RAW values at ISO 100. 12 bits could not count that
accurately. Even at ISO 200, it would cause an uneven histogram, with a
gap every so often. Electrons need to be oversampled by about 3x or so
to avoid erratic histograms (the effects, of course, are quite subtle,
but if you really were just counting photons, it could make a visible
difference in deep shadows pushed in PP). Of course, read noise make
exact photon counting impossible with any bit depth.

> Again, not doing well with 12-bit A/Ds. That is why canon has
> now announced a camera with 14-bit A/Ds. We'll see more of that
> in the future,

Again, probably marketing lies. The read noise is analog. The 1DmkIIIs
that Canon has released to reviewers has the same read noise, relative to
max signal at ISO 100, as the mkII does.

You greatly overestimate the role of bit depth. Its effect is rather
subtle in the ranges we're talking about.

>> The real world does not match your boogey-man stories of read noise
>> problems with small pixels.

> The real world marches to a complete description of physics,
> not a narrow view to push an agenda.

The real world has small pixels that bin down to better pixels than the
big DSLR pixels, and are better also, unbinned, with the same
magnification of the sensor surface. So to say that larger pixels are
needed for IQ and SQ is nonsense.


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John P Sheehy <JPS(a)no.komm>
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From: John Sheehy on
Lionel <usenet(a)> wrote in

> On Sat, 17 Mar 2007 21:40:31 GMT, John Sheehy <JPS(a)no.komm> wrote:

>>Lionel <usenet(a)> wrote in

>>> Exactly. And he's also neglecting to account for (in his analogy),
>>> the drops that hit the lips of the smaller containers, & are lost,

>>Apparently, not many photons are lost with the 1.97u pixel pitch in my
>>FZ50. It captures almost exactly the same amount of photons per
>>square mm at ISO 100 saturation as the 1DmkII! Even photons that get
>>close to the edge may still go into a well,

> No, that's not how photodiodes work. Those photons are lost.

Which ones are lost? You made a very vague reference to drops on the
edge; I can only guess the scenario in which they occur. I don't know
how thick the walls are in your mind, etc. I get the feeling that you
meant imply "in the worst and most impractical manner possible". The
fact is, current tiny-sensor pixels are not losing many photons the way
they would in your horror story.

>> whether it is the next one over is
>>irrelevant, as the resolution is still more precise than if you had
>>large pixels, where any collected photon could have been in any of a
>>number of pixel wells on a finer-pixel-pitch sensor.

>>You're talking boogey-men. Talk facts, from real world stuff, please.
>> No cultish hand-waving.

> And you're calling names, not supplying facts.

I hope I didn't offend any boogey-men, but then again, they *want* to
scare you.

I am supplying facts about various cameras and how much noise they have,
and how they bin. You're just making up worst-case scenarios.

>>> (equivalent to the extra fill-factor loss), & the fact that the
>>> smaller containers will fill & overflow more easily at points with
>>> heavy exposure (which is equivalent to lowered well capacity in the
>>> smaller photodiodes, thus a reduced maximum photon capacity).
>>You're getting very funny now. Go back and look at what you just
>>wrote; you just complained about resolution!
> I did? Where?

You complained about a small container filling up, while its neighbors

>>What if there was a pattern of extra drops in every second row of
>>small containers? How would you see that with the larger containers?

> How would you see it if you're binning all these drops? - You /are/
> still talking about binning all these samples, aren't you?

Binning is optional. It's saves space on memory cards, and speeds up
their writes.


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John P Sheehy <JPS(a)no.komm>
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From: John Sheehy on
Lionel <usenet(a)> wrote in

> On Sat, 17 Mar 2007 23:41:44 GMT, John Sheehy <JPS(a)no.komm> wrote:

> DR depends on a number of factors, of which noise is only one. At the
> analog level, dynamic range is the difference between the smallest
> detectable signal & the largest measurable signal.

Which is in the same ratio with or without microlenses.


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