From: Doug McDonald on
John Sheehy wrote:
r
> area, which is much more relevant. Noise per pixel is just a myopic,
> "missing-the-forest-for-the-trees" academic curiosity with no direct
> relationship to practical photography, in terms of pixel density.
>
> Think hard about this, if you think large pixels give better images, for
> shot noise reasons:
>
> Imagine that you had 16 square containers, and you gave them to an
> assistant, to place in a tight 4x4 array out in a field, to measure
> rainfall during a certain period of time.
>
> Your assistant is me, and I decide to replace the 16 containers with 64
> square containers, 4 of which fit in the same space as 1 of yours. I come
> back to you with a list of results from 64 smaller containers, instead of
> the 16 you asked for. The list is longer, and the total count is the
> same as it would be if I used your original containers, but have I
> created any *NOISE*?
>
> Of course not, but many people believe so for capturing photons instead
> of raindrops!
>


That last sentence is WRONG. That said, it is wrong only because
of read noise, which is of the order of 3 to 5 electrons. If the
actual signal is bigger than say 100 photoelectrons the read
noise becomes negligible. Nevertheless, at the very lowest signal levels,
your argument is wrong in the real quantized world.

Doug McDonald
From: John Sheehy on
Lionel <usenet(a)imagenoir.com> wrote in
news:pl1ov2pel4om9dcke66kn1k21ubfld0iv3(a)4ax.com:

> On Sat, 17 Mar 2007 15:14:50 GMT, John Sheehy <JPS(a)no.komm> wrote:

>>At the pixel level.

> Pixel level is what we're discussing here.

Obviously; you are a member of the "Pixel as an end in itself" club.

I'm saying that pixels don't determine noise by themselves. Their
spatial magnification is another factor in real-world noise strength.

> If you don't actually
> understand how photodiodes or sense amplifiers work, & aren't
> interested in finding out, there's not really a lot of point in you
> arguing with people about them.

I don't know all the details of the process, but I *CAN* read the
evidence of current cameras, which says that read noise does *NOT*
deteriorate as much with small signals in the real world, as you boogey-
man stories suggest.

>> I don't worship pixels. I use them to record
>>images.

> Then why are you arguing about them?

I'm arguing against them as ends in themselves. My interest is in the
subject and the image, and pixels are only one of the factors. The noise
of an image, and the dynamic range of an image, are not directly related
to the noise of the pixels. That is just a lot of a priori nonsense that
has been repeated so many times that people believe and defend it.

> Drop the thead, & discuss the
> images instead.

I have been, all along. Are you really that dense?

>> having more of them, with slightly less accuracy each, has more
>>useful resolution information about the subject, and when binned, has
>>less read noise.

> Except that it doesn't. It's entirely possible that the sensor you're
> talking about produces a better image than the other sensor of the
> same size that you're comparing it too, but if so, it won't be because
> the one you like has smaller pixels, it'll be because it has better
> photodiodes, sense amps, ADCs, software, (or some combination of the
> above) than the other sensor.

Hardly. Extra resolution is worth a little more noise, even if there is
more noise at the image level. At the pixel level, noise can increase
with smaller pixels, and still result in lower image noise.

>> This is what is happening now in the real world,
>>despite your boogey-man stories. Did you read the figures for the
>>FZ50 pixels binned to DSLR size? 8.1 electrons of read noise, out of
>>a max of 43,200 photons, at ISO 100. That is what is real, right now.
>> Forget your boogey-man read noise horror stories.

No comment on this? It is the jist of my argument.

>> and as useage
>>changes, so does the electronics. Think about Canon high ISO - very
>>low read noise for small signals; less noise in electrons at higher
>>ISOs, where there are *SMALLER SIGNALS*.

> Oh dear. Please tell me you don't seriously think that the process is
> as simple as that.

You'd love to believe that, wouldn't you? This is an *EXAMPLE* of how a
lower signal can is amplified with less added (read) noise. An
*EXAMPLE*, to show that there isn't a strict real-world correlation, and
all you can think of responding is to try to belittle me as if I were to
say, "the higher the gain, the less noise added, always".

> There are actually a number of possible
> explanations for the increased sensitivity & higher noise in Canons
> higher ISO modes, the most obvious being that they simply turn up the
> gain in the read amps, which is also simple, & has the advantage that
> it's compatible with the laws of physics.

That sounds like a genie wish, and you accuse *me* of naive simplicity.
What a joke.

>> Your generalization falls apart in
>>reality.

> You've never designed an amplifier, have you? ;^)

You've never looked at existing products, have you? The read noise in
small-pixel cameras, binned down to large pixels, can be less than in
native big pixels. This is a fact, and you seem to want to avoid it, and
cling to worst-case scenarios that are far from current reality.

--

<>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<>
John P Sheehy <JPS(a)no.komm>
><<> <>>< <>>< ><<> <>>< ><<> ><<> <>><
From: Doug McDonald on
David J. Littleboy wrote:

> How much of an issue is fill factor _in the presence of microlenses_? I'd
> think that microlenses would mean that fill factor in the silicon itself is
> much less of an issue. (This is the main issue I'd like to see addressed.)
>

I think that that depends on the f/number of the camera lens. At f/1.2
microlenses that really help are going to be hard to fabricate (they reach their
ultimate limit at about f/0.25 using lenses of diamond or cubic zirconia).

Do current microlenses function fully if the light is f/1.2 coming in
(and, at the corners, at an angle.)?

Doug McDonald
From: Doug McDonald on
Doug McDonald wrote:
> John Sheehy wrote:

>>
>> Your assistant is me, and I decide to replace the 16 containers with
>> 64 square containers, 4 of which fit in the same space as 1 of yours.
>> I come back to you with a list of results from 64 smaller containers,
>> instead of the 16 you asked for. The list is longer, and the total
>> count is the same as it would be if I used your original containers,
>> but have I created any *NOISE*?
>>
>> Of course not, but many people believe so for capturing photons
>> instead of raindrops!
>>
>
>
> That last sentence is WRONG. That said, it is wrong only because
> of read noise, which is of the order of 3 to 5 electrons. If the
> actual signal is bigger than say 100 photoelectrons the read
> noise becomes negligible. Nevertheless, at the very lowest signal levels,
> your argument is wrong in the real quantized world.
>


I should add that in the limit of really small pixels, I suppose that
a designer could get the read noise well below 1 electron. When that happens,
you really DO reach the limit where smaller pixels are better (except
for fill factor arguments and the associated factor of microlenses.) It IS
possible to get semiconductor read noise below one electron, it just hard to
do very fast. With read noise well below one electron, and enough analog gain, amplifier noise
at low signals (streaks) can be processed way.

Doug McDonald
From: Roger N. Clark (change username to rnclark) on
John Sheehy wrote:
> Think Canon high ISO. Less noise in electrons, at higher gain. That's
> real world. The small-pixel cameras tend to have very good read noise at
> ISO 100, and poor amplifiers for high ISO; worse than pushing. Better
> can be done.

John, you are confusing several things in your argument in this thread.
1) Small pixel size cameras are at near unity gain at low ISO.
(for other readers, unity gain ISO is the ISO where 1 electron
equals one bit in the A/D converter). ISO's above unity gain
ISO is nothing more than "digital ISO." You essentially gain
nothing at higher ISOs as you can't measure a fraction of an electron.

2) Current DSLR cameras, like the Canon 20D, Nikon D50, and all other
DSLRs tested on my web pages and other people testing cameras
that I reference, ARE NOT SENSOR READ NOISE LIMITED AT LOW ISO.
You cite the high read noise of DSLRs at low ISO in electrons,
but that is an electronics limitation, NOT sensor limitation.

2a) As electronics improve, e.g we see that in the newly announced
canon 1D mark III with 14-bit converter, the low ISO noise
is reduced (according to Canon). Current scientific sensors
use 16-bit converters and achieve full well digitization with
good digitization of true sensor read noise. We'll see that
soon in coming DSLRs.

3) So what you are describing as lower read noise for binned small
pixel sensors, is really because the small pixel cameras are operating
at near unity gain ISOs, while the DSLRs are not. You are comparing
apples and oranges. (Interesting that DSLRs are not yet living up
to their low ISO potential! That is exactly what is shown in Figure 4 at
http://www.clarkvision.com/imagedetail/digital.sensor.performance.summary
where the P&S small sensors are below the 12-bit line and the DSLRs are
above the line indicating they are A/D electronics limited. Note the
data indicate the 1D Mark II and 5D would still be limited with 14-bit
converters, so the new 1D Mark III will still not quite live up to its
potential.)

4) DSLRs do not live up to their full low signal, low ISO potential because they
are A/D limited. Remember, all A/D conversions are accurate to +/- 1 bit.
A DSLR with 50,000 electron full well (e.g ~ Canon 20D) and a
12-bit converter digitizes 1 bit = 50000/(2^12 -1) = 50000/4095 = 12.2 electrons.
14-bit converter digitizes 1 bit = 50000/(2^14 -1) = 50000/16383= 3.05 electrons.
The read noise of the 20D sensor is 3.9 electrons. 14-bits is close to
digitizing that well but 12-bits is not (remember the +/- 1 bit added noise).


> Lionel <usenet(a)imagenoir.com> wrote in
> news:85vmv2ttgd09e4c6gq47mkrojj5pbme1kq(a)4ax.com:
>>> Noise per pixel or per unit of sensor area?
>> Both. At the macro level, it'll manifest as decreased dynamic range.
>> (See Roger Clarke's dynamic range vs ISO graphs for a real world view
>> of how this effect would appear.)

> John Sheehy wrote:
> Roger has a knack for testing one thing, and drawing conclusions about
> another. My tests show that low noise in big pixels is only an advantage
> when magnifying the *PIXELS* at the same size; not the subject - when the
> same focal length, Av, Tv, and ISO are used. Roger compares large
> pixel,large sensor to small pixels, small sensor, with equivalent FOV,
> and then draws conclusions about small *pixels*. That is wrong. The
> conclusions should be about small *sensors*.

The current crop of available cameras gives choices like:
8 megapixel P&S small sensor, small pixel camera versus
8 megapixel DSLR with large pixels. I provide data and information
for people to evaluate what they would lose in making a choice
between such cameras. THAT IS NOT WRONG.

For those new to my web sites, relevant web pages are at:
http://www.clarkvision.com/imagedetail/index.html#sensor_analysis

John, your assertion seems to be (correct me if I'm wrong):
Given two sensors of the same total size, one with large pixels,
and one with small pixels, the small pixel camera can deliver
equal quality images when the pixels are binned to the
size of the larger pixels. You then support this by saying
the low ISO read noise of DSLRs is larger than at high ISOs
and at low ISOs is larger than small pixel cameras.
You bolster your argument using currently electronics limited
DSLRs with fictitious small pixel sensors that have many times
the pixels currently available in any sensor (e.g. your 223 megapixel
sensor or something like that).

So with the fact that current 12-bit DSLRs must be limited in their
read noise at low ISOs and the fact that 16-bit scientific sensors
available (even amateur astronomers have such chips), it is a
matter of time before we see the DSLRs with lower effective read noise,
e.g. as is the case with the new 1D Mark III.
So, I think you should level the playing field and use true sensor
read noise in your calculations. When you do, you'll find this
is a common sampling problem and is encountered in science all the
time. The noise in any one measurement has an error. The smaller
the signal, the lower the signal to noise ratio of that measurement.
Adding multiple samples improves as the square root of the number
of samples.

So in the level playing field of equal read noise in both large and
small sensor, the small sensor binned up to the large sensor will have
read noise higher by the square root of the number of pixels binned.

If you want to try the experiment with current cameras, use an
ISO where the DSLR is not electronics limited, like ISO 800.
You'll see that the small pixel camera can actually come close
in binned images quality, but noise AT BEST at the low end will be worse
by square root number of pixel binned. Fill factor using edge
effects will actually make the small pixels worse overall.
How much worse depends on a number of factors, all quite predictable
if we knew what the parameters were (like the fill factors).

Then in the real world, you earlier complained about fixed pattern noise.
The practical problem with your idea is that for the small pixel
camera to compete with the large pixel camera, the fixed pattern
noise would have to be considerably less than the fixed pattern noise
in the large pixel camera, by at least the square root of the
number of pixels being binned (depends on the spatial frequency distribution
of the fixed pattern noise).

So, again, I resent you saying my data are wrong when you are applying it
to theoretical cameras that do not exist. My data and conclusions
are absolutely right for evaluating current cameras that DO EXIST and
that people are trying to choose and understand the differences between such
real cameras.

Roger