From: David J Taylor on 20 Jul 2008 02:12
Ray Fischer wrote:
> Not as stupid as are you. Noise per sesor area has no effect on one's
> photographs. Noise per pixel does.
>> It's the total image that counts,
> Which is affected by noise per pixel.
>> not the individual
>> pixel quality, and the total image is the product of per-area
>> quality and total area.
> Bullshit. "Per-area quality" is wholly irrelevant. Want proof? Find
> a photograph in the web and, without knowing what camera produced it,
> tell us how big the sensor was and what the noise per area was.
> You can't. Why? Because noise-per-area doesn't affect the final
Noise per sensor area affects the overall SNR - the pixel density affects
the spectrum of the noise - fine-grained noise versus coarse-grained
noise, if you like.
From: John Sheehy on 20 Jul 2008 02:26
ASAAR <caught(a)22.com> wrote in
> A convenient statement that makes no effort to show and quantify
> the differences. And we must, of course, assume that FZ50 and 5D
> circuit differences are negligible, because if not, you'd never have
> been tempted to use them to make such a meaningless, unfair
I'm getting really tired of repeating myself. You just snipped away in
your reply my previous response to the same thing. The ratio of pixel
densities between the 5D and the FZ50 is large. By the doomsday forecast
of high pixel density, the FZ50 pixels shouldn't stand a chance against
the 5D, but the 5D would perform worse than the 400D. Don't shoot me;
I'm just the messenger. Low pixel densities are inferior, unless they
are multi-transistor CMOS pixels at high ISOs.
Why do you try to create the pretense that I am purposely avoiding a
responsibility to compare any two cameras that you care to mention?
I can only compare cameras that I have in my posession, with any degree
of accuracy. Other people's JPEGs are worthless. Other people's RAWs
are worthless, if lighting and exposure aren't accurately controlled.
Other people's RAWs are worthless if they use different focal lengths.
Give me any camera you want, and I will compare its pixels and density to
those of my FZ50 or G9. I have nothing to hide; no fear of the truth.
If I wanted to bias my experiment, as I've been accused, I would have
used a worse choice DSLR for ISO 100 shadow SNR.
Oh, and if certain pairings are so important to you, why can't you do
them yourself and show us the results?
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John P Sheehy <JPS(a)no.komm>
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From: Ray Fischer on 20 Jul 2008 03:48
David J Taylor <david-taylor(a)blueyonder.neither-this-bit.nor-this-bit.co.uk> wrote:
>Ray Fischer wrote:
>> Who cares? People don't care about "pixel densities". People care
>> abut noise per pixel, not noise per sensor area.
>So why buy a 12MP APS-C DSLR over a 6MP APS-C DSLR if people don't care
>about pixel densities?
From: Bob Newman on 20 Jul 2008 03:49
On Jul 20, 2:26 am, ejmartin <ejm_60...(a)yahoo.com> wrote:
> On Jul 19, 8:15 pm, John Sheehy <J...(a)no.komm> wrote:
> > "Roger N. Clark (change username to rnclark)" <usern...(a)qwest.net> wrote innews:48815D3C.2030803(a)qwest.net:
> > > John Sheehy wrote:
> > >> Then again, I'm
> > >> the guy who got 100% on all his math and most of his science tests,
> > >> without studying, so maybe I'm expecting too much.
> > > So what happened? I showed you some of your math and conceptual
> > > errors but you failed to recognize them.
> > I haven't read your posts yet, except for a couple. It takes a long time
> > to reply to one of your longer posts, with all the tangents you go on
> > acting as if I was implying things that I wasn't.
> > The things I've seen of yours in other people's quotes have been off the
> > mark, and your 4 electron read noise does *NOT* exist in DSLR pixels at
> > base ISO. It's a fantasy. 6 or 8 FZ50-like chips fabbed into a larger
> > wafer with parallel readout is more imminently possible and realistic.
> > --
> John, if you look at any circuit diagram for 4T CMOS active pixel
> sensors, and compare it to any circuit diagram for a low-noise
> amplifier, you will see that there is no amplification going on in the
This isn't exactly the case. That source follower is an amplifier, but
it's amplifying the current, not the voltage. As it happens, that's
pretty much equivalent. The charge/voltage gain is supplied by the
capacitance, as Eric explained. However, since it still only contains
the photo (and shot noise) electrons, it's ability to drive the
downstream stages ins not big. The source follower amplifies the
current, introducing some noise along the way. I think that noise is
voltage, not charge dependent, but I could be wrong.
> Eric Fossum pretty much dismissed your notion that there is
> special circuitry at the photosite that is ISO dependent, in the
> thread he started at DPR. There is no ISO dependent structure at the
There are circuits that can do this, by changing the cell capacitance,
either by switching in and out capacitors or changing the DC bias on
the cell, and I was informed by one DPR correspondent that Canon has a
patent on this. However, I'm pretty sure no current camera does it,
and Eric seems to confirm.
> photosite. Therefore, in its contribution to read noise, an *upper*
> bound on the photosite read noise is the lowest read noise (in
> electron equivalents) as a function of ISO. This occurs at high ISO.
> The advantage of FZ50-like pixels at base ISO has little to do with
> pixel properties. Since there is nothing going on at the pixel that
> is ISO dependent, one can infer that the pixel read noise is *at most*
> the read noise at high ISO (in electron equivalents). That is 3-4
> electrons for the best pixels, across the spectrum from small to large
> pixels and relatively insensitive to pixel pitch.
I don't think we've positively established that the front end read
noise is truly electron referenced. I would like to see an explanation
of how it is so. The classical electronics says it isn't, I think we
have to look at the device physics and quantum effects to understand
more (i.e. what is the noise contribution of a single electron charge
on the gate of the source follower)
From: David J Taylor on 20 Jul 2008 04:16
Ray Fischer wrote:
> David J Taylor
> <david-taylor(a)blueyonder.neither-this-bit.nor-this-bit.co.uk> wrote:
>> Ray Fischer wrote:
>>> Who cares? People don't care about "pixel densities". People care
>>> abut noise per pixel, not noise per sensor area.
>> So why buy a 12MP APS-C DSLR over a 6MP APS-C DSLR if people don't
>> care about pixel densities?
Which suggests that you /do/ care about the pixel density....