From: Roger N. Clark (change username to rnclark) on
Lionel wrote:
> On Sat, 17 Mar 2007 13:22:21 -0700, "Roger N. Clark (change username
> to rnclark)" <username(a)qwest.net> wrote:
>
>> 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)
>
> Correction: You mean one count, not one bit.

By 1-bit I implied the least significant bit. In remote sensing science,
this is called a BN (Data Number).
Roger
>
>> 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).
>
> Exactly.
>
From: Roger N. Clark (change username to rnclark) on
Paul Furman wrote:
> Roger N. Clark (change username to rnclark) wrote:
>> 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).
>
> Aughhhh, I hate that I can't understand these discussions.
>
> Unity Gain ISO - the ISO where 1 electron = 1 bit in the A/D converter

Yes, the smallest integer interval. In a 12-bit A/D, there are
2^12-1 levels = 4095. So the finest interval that is recorded is
max signal into the A/D converter / 4095.

> A/D Converter - analog to digital, where electrons are assigned numbers

Yes. And digital converters always have an accuracy of +- one number.
So if the signal is 2/4095 of full signal, the answer the A/D will give
is sometimes 1, sometimes 2, or sometimes 3.

> So unity gain ISO is where there isn't a rounding error problem.
> Read Noise is the rounding problems, higher bit depth in the raw file
> lessens read noise.

Yes, assuming one can actually "see" one electron (and in electronic
sensors, the noise is only a few electrons, so there is really no benefit
to gains higher that digitizing one electron. It's really pretty impressive
when you think about it. We are buying, for a few hundred dollars,
devices (digital cameras) that directly detect quantum processes!

> P&S cameras don't have this problem because there are so few electrons,
> they are easy to count?

Effectively, yes. They capture so few photons that 12 bits (4095 levels)
adequately records the highest signals to the smallest signals with the
few electron noise. Current electronic sensors, CCDs or CMOS, capture
at most about 1000 to 2000 photo-electrons per square micron.
So a 2 square micron CCD fills up with electrons at only 4,000 to
8,000 electrons. 8000/4095 = 1.95 electrons per number out of
the A/D converter. But a large pixel DSLR can have 60+ square micron
collection area. For example the 1D mark II stores a maximum of about
80,000 electrons (ISO 50), so the 12-bit A/D converter gives
80,000/4095 = 19.5 electrons per data number. If you boost the
gain to a higher ISO, so you look at only the bottom 8,000 electrons,
then the A/D records 1.95 electrons per number, like the small
P&S camera, but at a much higher ISO. When you boost gain
to so one number in the A/D conversion is equivalent to 1 electron,
that is the unity gain ISO. That is a factor of more than
16 from current small pixel P&S cameras to large pixel DSLRs,
and is the fundamental reason why small sensor cameras have poor
high ISO performance, and why they always will relative to their
larger cousins.

> Does it really matter if there are minor rounding errors? Is it really
> noise because colors are off by 1 bit? Relevant noise is random wack
> hair-brained colors, not minute color shifts, right?

Noise that we view is mostly due to intensity variations.
Noise due to color shifts is called chrominance noise and is less
bothersome. Noise in bright parts of a scene are not objectionable
to most viewers, but the noise becomes more obvious in night scenes
or in shadow detail. For example, look at Figure 5 on this page:
http://www.clarkvision.com/imagedetail/does.pixel.size.matter2
The Canon S70 image looks pretty noisy, especially in the dark areas,
and that is due to a few electron noise. So 1 bit noise is usually
not a factor unless one is pushing limits (like is done in
high ISO action photography, night and low light photography).

> What is Dark Current?

All electronic sensors have some electrons that leak into the
well with the other electrons from the converted photons.
The dark current amount is temperature dependent and that adds noise
equal to the square root of the number of electrons accumulated
from the dark current over the exposure. For most modern
digital cameras with exposures less then a few tens of seconds,
dark current is negligible. For long exposure of
minutes it can become dominant over read noise.

> What's this business of clipping at blackpoint before setting gamma?
> That means you can set the blackpoint? How could there be negative
> number? Why?

Because there is noise all signals, e.g. read noise, the natural
fluctuations can send a measured signal to negative voltage.
Manufacturers usually set a small offset in the electronics
voltage to compensate. Lets say the sensor put out 1 volt on the
output amplifier to the analog-to-digital (A/D) converter. Manufacturers
add a small negative voltage, like 0.02 volts so the A/D converter
digitizes from -0.02 volt to 1 volt. Thus 0 light on the sensor
gives about number 100 in the output raw file. Some raw converters
subtract off that level, but some values will be less than 100, and
in the subtracted image, values would be clipped at zero.

> I don't get the charts against pixel pitch. It doesn't matter because
> there could be some efficiency or inefficiency in the layout, the only
> thing that matters is full well electrons, right?

Actually, what matters is:
1) quantum efficiency of converting photons to electrons
(typically in the 30 to 50% range in modern digital cameras,
and that is very good),
2) active area to convert photons to electrons (currently effectively
in the 80% range although manufacturers do not generally
publish that number (Kodak does on their sensors),
3) the full well capacity to hold those electrons.

Quantum efficiency is similar for current consumer devices, so
within a factor of two they are pretty much the same. Full well
capacity is correlated to pixel pitch, as is active area.
Full well capacity is about 1000 to 2000 electrons per square
micron. The vertical scatter in the pixel pitch plots you refer to are
mostly due to the variations in active area, full well capacity
and quantum efficiency between devices.

> Signal to Noise seems clear enough, shoot a gray card & count the pixels
> that come out some color other than gray.

Not quite. Not a color, but an intensity in each red, green or
blue channel.

> What is the significance of raw noise versus final bayer interpolated
> RGB values unless you are doing binning to interpolate by greatly
> shrinking the pixel count? (if I understand the term 'binning' correctly)

The raw conversion with Bayer interpolation is variable. Some converters,
like the Canon converter do minimal sharpening and effectively average
pixels, reducing noise by about 1.5x. Other converters (in their default
settings) attempt to increase apparent spatial detail but at the
expense of increased noise. The Rawshooter Essentials is one
such example (technology now in Photoshop CS3 beta), and does very
well in my experience. It is nice to have the high signal to noise
ratio that large pixels give to play the game in raw conversion:
do I want a lower noise lower resolution image, or more detail
at the expense of noise? If the signal-to-noise ratio is high
to begin with, you can afford to push for more apparent
spatial detail. You don't have that luxury with smaller pixels
and the lower signal-to-noise ratios they give.

> And who cares what the characteristics are before white balancing?
> Nobody is using un-whitebalanced images and the basic WB is fairly
> dramatic in any lighting. I can see how these things make the math clean
> but I don't see how they are necessarily relevant in the final product.

Yes, I basically agree. One must have adequate S/N to white balance,
however. For example, there are very few blue photons from
an incandescent lamp, so after white balancing noise in the blue
can be quite large. In that case it might be better to use a color
correction filter on the lens and a longer exposure to get more blue
photons.

> Ah, my head hurts... am I understanding?

I think so. It's those who don't know what question to ask that
are probably not understanding (unless of course they completely
understand).

Very good questions. I'll probably develop this into a web page and
add it to my sensor analysis section.

Roger
From: John Sheehy on
"Roger N. Clark (change username to rnclark)" <username(a)qwest.net> wrote
in news:45FDA453.7060303(a)qwest.net:

> It is also used in scientific work. Thus it is well understood,
> and the claims being made here about equaling or bettering
> a larger pixel sensor have never been demonstrated. The one
> example being used in this thread to support the claim is
> a restricted case where the larger pixel sensor is A/D limited,
> not sensor performance limited. If John would do the test
> with the same two sensors at a higher ISO where the
> large pixel sensor was not A/D limited, he would come to the
> opposite conclusion.

The "one example" was at ISO 1600. Read noise is 3.34 electrons at ISO
1600 on the FZ50, and as I already said, I found afterwards that just
puxhing ISO 100 would have been better, with a read noise of 2.7 electrons.
3 2.7 electron FZ50 ISO 1600 pixels binned together will collect a max of
2700 electrons, with a read noise of about 8.1 electrons, quite comparable
to a DSLR. The best Canons are about half of that; shot noise is
significant in ISO 1600 shadows, however, and should be similar.

If you don't bin, you have 3x the linear resolution.

--

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John P Sheehy <JPS(a)no.komm>
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From: John Sheehy on
Rita � Berkowitz <ritaberk2O04 @aol.com> wrote in
news:12vrbb9f5nu3mac(a)news.supernews.com:

> John Sheehy wrote:
>
>> I am "jabbering" about the fact that you went way out of the context
>> of the conversation to create a scenario that involves microlenses
>> and affects dynamic range. The assumed normal context is uniform
>> microlenses, and all other things being equal. If you had something
>> unusual in mind like mixed well depths and mixed microlenses, you
>> should have said so. I never would have said that partial
>> microlenses would not affect DR of the system.
>
> The reality of all of this is if you were to use the world famous and
> legendary 58mm f/1.2 Noct Nikkor none of this would even be a concern.
> A great lens eliminates all of these problems, real or imagined.

Which problems? There are multiple layers going on here.

I'll assume you mean the long-lost topic of the the OP; low light (sorry, I
should have started a new thread).

Even with fast lenses, you hit limits, too. And do you really want to use
such a lens wide open? Focusing is difficult in low light, and the DOF at
f/1.2 is not very forgiving. Many fast lenses are optically compromised
wide open, too, even when focused, and tend to have lots of luminance roll-
off in the corners. I generally don't let my f/1.4 lenses shoot below
f/2.0. The lens you suggest, of course, may only have the DOF issue.

--

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

"John Sheehy" <JPS(a)no.komm> wrote:
> Lionel <usenet(a)imagenoir.com> wrote:
>
>> Not that I know of. But all the Canons already offer this amazing
>> 'binning' feature anyway - that's what you get when you set your
>> camera to JPEG mode, & use a JPEG size smaller than full resolution.
>> ;^)
>
> Well, downsampling and binning are a bit different. A downsampling should
> use a filter to remove frequencies that complicate and artifact the
> output.
> Binning is just a box effect, with the contents literally added together.

Binning will result in gross aliasing effects, which are very nasty with a
Bayer sensor. So it's probably not useful in real life.

Noise reduction (via a Gaussian blur) followed by downsampling wouldn't have
that problem.

David J. Littleboy
Tokyo, Japan