Yep, I didn't pay much attention to that footnote as it says "4 There are
other sources of noise that are beyond the scope of this article", so it
just mentions some other noise sources without actually discussing them.
...Wayne
-----Original Message-----
From: olympus
[mailto:olympus-bounces+wayne.harridge=structuregraphs.com@xxxxxxxxxxxxxxxxx
] On Behalf Of Chuck Norcutt
Sent: Wednesday, 23 September 2015 12:16 AM
To: Olympus Camera Discussion <olympus@xxxxxxxxxxxxxxxxx>
Subject: Re: [OM] Digital Noise
That's because you didn't read the footnotes to part 2
--------------------------------------------------------------
Footnotes
*1 The most salient noise sources upstream of ISO amplification include
reset noise, pixel amplifier noise, and dark current shot noise for longer
exposures (which we won't dive into). Reset noise originates from variations
in the voltages each pixel is reset to after a charge is read (ideally,
they'd all be reset to the exact, same voltage). These variations - that
show up as changes in intensity across neighboring pixels - can largely be
mitigated by a process called correlated double sampling (CDS), which
samples and subtracts the reset voltage at any pixel from the total voltage
due to exposure. It can almost be thought of as a dark frame subtraction,
and is very effective, lowering pixel-to-pixel variations, as well as
pixel-level amplifier noise, to the level of single electrons in modern
sensors. However, CDS isn't completely effective, and so a non-zero noise
component still remains.
For reasonable exposure lengths, any remaining reset and pixel amplifier
noise post-CDS are probably the largest sources of upstream read noise.
[Jump back to text]
*2 Downstream read noise includes all sources from and after, or 'downstream
of,' ISO amplification. This includes noise from the programmable gain
amplifier (set by your ISO setting), noise from the analog to digital
conversion process, as well as any noise introduced in the pathways between
all these electronic components in your camera.
[Jump back to text]
*3 How much of this light a camera is able to make use of depends on its
sensor efficiency, which is beyond the scope of this article. We mention it
here, though, because sensors with higher efficiencies will record more of
the available light during any given exposure: leading to less shot noise
and a slightly cleaner result in our exposure latitude test.
[Jump back to text]
*4 There are other sources of noise that are beyond the scope of this
article. Thermal noise builds up with long exposures, amplifier glow can
lead to 'hot spots' at the edges of your image, pixel-response
non-uniformity can lead to noise even in brighter areas of your image, and
quantization error further limits signal-to-noise ratio in shadows.
[Jump back to text]
-------------------------------------------------------------------------
Chuck Norcutt
On 9/22/2015 8:47 AM, Wayne Harridge wrote:
Thanks (I think) for that reference Moose.
Unfortunately I don't think it addresses increased noise from long
exposures, ok the relative shot noise increases when the amount of
light hitting the sensor is low and you tend to use longer exposures
when the light level is low, but this is also the time when you also
might opt for high ISO. No mention of increased noise at long
exposures due to heating of the sensor.
--
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