From: ejmartin on
On Jul 27, 2:57 pm, rfisc...(a)sonic.net (Ray Fischer) wrote:
> John Sheehy  <J...(a)no.komm> wrote:
>
> >rfisc...(a)sonic.net (Ray Fischer) wrote in news:488cb5bd$0$17200
>
> >> I suspect that the key problem is that it's not possible to read out
> >> the iso1600 image while continuing to expose the iso100 image.
>
> >There is only one exposure in his scenario; it is simply read out in two
> >prallel channels with different gain.
>
> Then you have a noisy image at iso100.  Not all the noise comes from
> the amplifier.  Much (maybe most) comes from the sensors themselves.  
> With few photos and then few electrons to read out there is more
> sensitivity to randomness.
>
> Of course you can increase the size of the sensors.  Making them much
> bigger improves their sensitivity and reduces noise at low light
> levels.  But the drawback there is you end up with a 2MP camera.
>
> --
> Ray Fischer        
> rfisc...(a)sonic.net  

One of the points of the demonstration is that, at low light levels at
ISO 100, most of the noise *does* come from the amplifier. If it
weren't so, then there wouldn't be any difference with the ISO 1600
image (shot with the exact same shutter speed and aperture) blended
in. Instead, there is a dramatic improvement. Note also that most of
the nasty line noises are an artifact of the amplifier, and disappear
to a large extent in the blended image.
From: John Sheehy on
rfischer(a)sonic.net (Ray Fischer) wrote in
news:488cd310$0$17182$742ec2ed(a)news.sonic.net:

> John Sheehy <JPS(a)no.komm> wrote:

>>There is only one exposure in his scenario; it is simply read out in
>>two prallel channels with different gain.

> Then you have a noisy image at iso100. Not all the noise comes from
> the amplifier. Much (maybe most) comes from the sensors themselves.

The noise in the sensor is a given. However, there is much more
read/digitization-related noise at base ISO, relative to absolute
signal, in advanced CMOS designs. The objective in Emil's desired
camera design is not to get less sensor noise, but less of the extra
noise caused by low gain, which severely restricts DR in cameras with
big pixels.

> With few photos and then few electrons to read out there is more
> sensitivity to randomness.

That might be an issue varying the exposure, but not varying the readout
gain.

> Of course you can increase the size of the sensors. Making them much
> bigger improves their sensitivity and reduces noise at low light
> levels.

So far, this sounds reasonable.

> But the drawback there is you end up with a 2MP camera.

Now, it sounds like you're talking about increased photosite size, but
you wrote about increased sensor size; two different things.

The reality of the situation is that for base ISO performance and DR, a
higher density of smaller photosites works better with the traditional
design. As I demonstrated in my OP, the higher pixel density does not
mean poorer performance. What Emil is trying to do is to get the
practical, area-based read noise low with big pixels, which is not
happening with current technology. With current technology, higher
pixel densities yield lower read noise at base ISO, higher DR, and
higher resolution for the image. Emil's suggestion could get the DR and
read noise a bit better than higher pixel densities can for base ISO,
but without their resolution benefit.

--

<>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<>
John P Sheehy <JPS(a)no.komm>
><<> <>>< <>>< ><<> <>>< ><<> ><<> <>><
From: ejmartin on
On Jul 29, 4:27 pm, John Sheehy <J...(a)no.komm> wrote:
> rfisc...(a)sonic.net (Ray Fischer) wrote innews:488cd310$0$17182$742ec2ed(a)news.sonic.net:
>
> > John Sheehy  <J...(a)no.komm> wrote:
> >>There is only one exposure in his scenario; it is simply read out in
> >>two prallel channels with different gain.
> > Then you have a noisy image at iso100.  Not all the noise comes from
> > the amplifier.  Much (maybe most) comes from the sensors themselves.
>
> The noise in the sensor is a given.  However, there is much more
> read/digitization-related noise at base ISO, relative to absolute
> signal, in advanced CMOS designs.  The objective in Emil's desired
> camera design is not to get less sensor noise, but less of the extra
> noise caused by low gain, which severely restricts DR in cameras with
> big pixels.
>
> > With few photos and then few electrons to read out there is more
> > sensitivity to randomness.
>
> That might be an issue varying the exposure, but not varying the readout
> gain.
>
> > Of course you can increase the size of the sensors.  Making them much
> > bigger improves their sensitivity and reduces noise at low light
> > levels.
>
> So far, this sounds reasonable.
>
> > But the drawback there is you end up with a 2MP camera.
>
> Now, it sounds like you're talking about increased photosite size, but
> you wrote about increased sensor size; two different things.
>
> The reality of the situation is that for base ISO performance and DR, a
> higher density of smaller photosites works better with the traditional
> design.  As I demonstrated in my OP, the higher pixel density does not
> mean poorer performance.  What Emil is trying to do is to get the
> practical, area-based read noise low with big pixels, which is not
> happening with current technology.  With current technology, higher
> pixel densities yield lower read noise at base ISO, higher DR, and
> higher resolution for the image.  Emil's suggestion could get the DR and
> read noise a bit better than higher pixel densities can for base ISO,
> but without their resolution benefit.
>

More than a bit better. I think we're agreed that FZ50 pixels are
competitive in light collection per unit area with D3 pixels. If one
compares read noises that pertain to my suggestion, the D3 has about
5e- read noise with 8.45µ pixels while the FZ50 has 3.3e- read noise
with 1.97µ pixels (according to your figures; it was more like 5e- in
the raw file I analyzed). The read noise/area figure of merit divides
the read noise in electrons by the pixel pitch, and those figures are
5/8.45=0.6 for the D3, and 3.3/1.97=1.7 for the FZ50, about 1.5 stops
worse for the small pixels. In other words, the target you've been
aiming at is the low ISO performance of big pixels limited by the DR
of components other than those pixels; once freed from that
restriction, their performance exceeds the small pixels in terms of
dynamic range. The results are consistent with what both you and
Roger have been saying -- the 12.5 stop DR of the FZ50 when scaled to
the D3 pixel size outperforms the D3 at base ISO in DR, but the D3
pixels freed of their downstream circuits' limited DR have 14 stops or
perhaps a bit more and thus quite a bit more DR than the FZ50 pixels
on a per area basis.

But I think the DR numbers don't reveal a different and equally
important issue -- in your more recent comparison

http://forums.dpreview.com/forums/read.asp?forum=1018&message=28760503

there is lots of line noise with the FZ50, so read noise is having a
substantial impact on image quality (which indeed makes it interesting
to see whether the G9 does better, as you hint it should). On the
other side, the dual amplification/blended image sample I presented
shows extremely little in the way of line noise, even 12-13 stops down
from raw saturation, and a range of about 10-11 stops where the image
is shot noise limited.

So the conclusion I am coming to is that (as Roger had been claiming
for some time) the issue is really a tradeoff of low noise/sensitivity
favoring big pixels, versus resolution favoring small pixels. And the
issue becomes what is the optimal tradeoff for a given application; I
can see some applications (landscape comes to mind) favoring small
pixels, while others (photojournalism eg) favoring large pixels.