What Creates Noise/Grain At Higher ISO Speeds?

"Matt" wrote in message
...
What is the actual reason why photos shot at higher ISO speeds create
noise/grain?

In film, I can understand that to produce films which is more sensitive to
light, the chemistry may be different, but why is this evident with
digital?

If you look here, noise is very evident in this digital shot, but I am
puzzled why it occurs:

What happens in a digital camera is that light entering the sensor creates a
voltage at each site. If we assume that any sensor site peaks at say 1
volt, then that's the point of white saturation. In a proper exposure, it
would be nice to have no voltages at 1 but at least some close to 1 for the
very brightest details in the scene. Now, suppose the scene is darkish and
the highest voltage recorded is only 1/10 of a volt. Not a big problem in a
digital camera, since amplifiers with a multiplication factor of 10 (called
a voltage gain of 10) can beef up all the sensor signals before the
information is converted to digital form. However, the gain of 10 also
multiplies the noise by a factor of 10. So, when you shoot a scene at a
high ISO, you are using high gain and seeing noise that you ordinarily would
not see. Hope this helps. Tune an FM radio receiver to a frequency where
there is no station and the hiss that you hear is aural noise. Look at a TV
displaying a weak signal and the grain that you see is noise.

Alan Browne wrote:

Matt wrote:

What is the actual reason why photos shot at higher ISO speeds create
noise/grain?

In film, I can understand that to produce films which is more
sensitive to
light, the chemistry may be different, but why is this evident with
digital?

If you look here, noise is very evident in this digital shot, but I am
puzzled why it occurs:
http://www.pbase.com/jps_photo/image/36330285/original


Film suffers larger physical grain (dye clouds) in the plane of the film
(call it x,y) as the ISO goes up. Faster speed = larger 'grain' or dye
clouds. This results in the grain that you see. It's not really that
cut and dry, but the end result is coarseness in the negative image that
we call "grainy".

In digital, the x,y is constant regardless of the sensitivity... the
grain size will always be the same for a given sensor. However, the
noise level will be different from pixel to pixel, and this difference
is amplified by the increasing sensitivity... that is what gives the
noisy 'grainy' look on high ISO digital shots. The 'grains' remain the
same size, but there is a greater dynamic difference between them due to
noise. This is more complex than that, as each pixel that you see from
most SLR's is a composite of several pixels of different color. The
noise response in, eg, red, green and blue channels is different for
each color and this further makes pixel to pixel noise differences more
apparent in the resulting end pixel.

Hope that helps.

While the responses so far are close to the answer, what I've seen
is not quite complete. Noise is due to several things, including
read noise, thermal/shot noise, and photon noise. Read and
thermal noise will dominate in longer exposure images. In short
exposure images, photon noise can dominate (probably depends
on the camera). Newer cameras, and especially DSLRs, which
use low noise electronics, most will likely be photon noise
limited for daytime scenes (exposures up to at least a fair
fraction of a second.

Photon noise limit: what does it mean? When counting photons,
the noise is the square root of the number of photons
counted. This is a fundamental physical limit.
For electronic sensors, the number of photons
counted is dependent on the quantum efficiency (which is
very high), and the size of the pixel (proportional
to the number of photons that can be collected per unit
time), and the electron well capacity of the device
(also generally scales with the size of the pixels).

A high-end camera, like the Canon 1D Mark II, which has
pixel spacing of about 8 microns, has a full well
capacity of a little over 52,000 electrons. That means
the noise at maximum signal is sqrt(52000) = 228, and the
maximum signal to noise is 52000/sqrt(52000) = 228.
At lower signal, the noise is less, but so is the
signal. For example, at half capacity, 26000, the signal
to noise is 161.

For a camera with smaller pixels, and smaller full well,
the total noise is less, but so is the signal. Let's assume
a sensor with 4-micron pixels and a full well capacity
of 13,000 electrons. The maximum signal to noise is
only 114.

I've described the signal to noise of the 1D Mark II at:
http://clarkvision.com/imagedetail/d...ignal.to.noise
and show that it is working at the photon noise limit.
In particular, wee figure 2. This means
photon nose is the fundamental limit for that sensor.
The only way to improve on these values is to increase
the pixel area and full well capacity. Like I said above,
other digital cameras are likely working at the photon noise
limit too. The smaller sensors will never improve on this
fundamental limit, at least as we understand physics today.

Film is probably also photon noise limited, but since its
quantum efficiency is very low, the signal to noise is much
lower too. See figure 1 on the above page. For faster film,
the larger grains are collecting fewer photons, so one would
expect that the signal to noise would drop. My web page
shows noise levels ~4 times worse for film compared to
electronic sensors at similar ISO values, consistent with
the lower quantum efficiency.

So, if you want lower noise images, buy a camera with a
larger sensor to collect more photons for a given exposure.

Roger Clark
Photos, other digital info at: http://clarkvision.com

Alan Browne wrote:

Matt wrote:

What is the actual reason why photos shot at higher ISO speeds create
noise/grain?

In film, I can understand that to produce films which is more
sensitive to
light, the chemistry may be different, but why is this evident with
digital?

If you look here, noise is very evident in this digital shot, but I am
puzzled why it occurs:
http://www.pbase.com/jps_photo/image/36330285/original


Film suffers larger physical grain (dye clouds) in the plane of the film
(call it x,y) as the ISO goes up. Faster speed = larger 'grain' or dye
clouds. This results in the grain that you see. It's not really that
cut and dry, but the end result is coarseness in the negative image that
we call "grainy".

In digital, the x,y is constant regardless of the sensitivity... the
grain size will always be the same for a given sensor. However, the
noise level will be different from pixel to pixel, and this difference
is amplified by the increasing sensitivity... that is what gives the
noisy 'grainy' look on high ISO digital shots. The 'grains' remain the
same size, but there is a greater dynamic difference between them due to
noise. This is more complex than that, as each pixel that you see from
most SLR's is a composite of several pixels of different color. The
noise response in, eg, red, green and blue channels is different for
each color and this further makes pixel to pixel noise differences more
apparent in the resulting end pixel.

Hope that helps.

While the responses so far are close to the answer, what I've seen
is not quite complete. Noise is due to several things, including
read noise, thermal/shot noise, and photon noise. Read and
thermal noise will dominate in longer exposure images. In short
exposure images, photon noise can dominate (probably depends
on the camera). Newer cameras, and especially DSLRs, which
use low noise electronics, most will likely be photon noise
limited for daytime scenes (exposures up to at least a fair
fraction of a second.

Photon noise limit: what does it mean? When counting photons,
the noise is the square root of the number of photons
counted. This is a fundamental physical limit.
For electronic sensors, the number of photons
counted is dependent on the quantum efficiency (which is
very high), and the size of the pixel (proportional
to the number of photons that can be collected per unit
time), and the electron well capacity of the device
(also generally scales with the size of the pixels).

A high-end camera, like the Canon 1D Mark II, which has
pixel spacing of about 8 microns, has a full well
capacity of a little over 52,000 electrons. That means
the noise at maximum signal is sqrt(52000) = 228, and the
maximum signal to noise is 52000/sqrt(52000) = 228.
At lower signal, the noise is less, but so is the
signal. For example, at half capacity, 26000, the signal
to noise is 161.

For a camera with smaller pixels, and smaller full well,
the total noise is less, but so is the signal. Let's assume
a sensor with 4-micron pixels and a full well capacity
of 13,000 electrons. The maximum signal to noise is
only 114.

I've described the signal to noise of the 1D Mark II at:
http://clarkvision.com/imagedetail/d...ignal.to.noise
and show that it is working at the photon noise limit.
In particular, wee figure 2. This means
photon nose is the fundamental limit for that sensor.
The only way to improve on these values is to increase
the pixel area and full well capacity. Like I said above,
other digital cameras are likely working at the photon noise
limit too. The smaller sensors will never improve on this
fundamental limit, at least as we understand physics today.

Film is probably also photon noise limited, but since its
quantum efficiency is very low, the signal to noise is much
lower too. See figure 1 on the above page. For faster film,
the larger grains are collecting fewer photons, so one would
expect that the signal to noise would drop. My web page
shows noise levels ~4 times worse for film compared to
electronic sensors at similar ISO values, consistent with
the lower quantum efficiency.

So, if you want lower noise images, buy a camera with a
larger sensor to collect more photons for a given exposure.

Roger Clark
Photos, other digital info at: http://clarkvision.com

In message ,
Alan Browne wrote:

Nah. Don't worry about assorted crankies.

No one is being cranky. I replied as I did because he may have thought
that it was an entire image shrunk to web size.

Noise of that magnitude with strong sharpening in a pixel-for-pixel crop
is to be expected.
--


John P Sheehy

Hi

A higher ISO speed allows you to shoot better pictures in low light
conditions. A higher ISO speed allows you to use a faster shutter
speed and/or a smaller aperture in a given light level.

The drawback of higher ISOs are as follows:
- Increased noise
- Larger file sizes — you'll get far fewer shots per memory card at
ISO 800 than at ISO 100.
- Reduced shadow detail and sharpness

The same problem exists for analog cameras when you use fast film.

Regards
Gary Hendricks
www.basic-digital-photography.com


"Matt" wrote in message ...
What is the actual reason why photos shot at higher ISO speeds create
noise/grain?

In film, I can understand that to produce films which is more sensitive to
light, the chemistry may be different, but why is this evident with digital?

If you look here, noise is very evident in this digital shot, but I am
puzzled why it occurs:
http://www.pbase.com/jps_photo/image/36330285/original

wrote:

In message ,
Alan Browne wrote:


Nah. Don't worry about assorted crankies.


No one is being cranky. I replied as I did because he may have thought
that it was an entire image shrunk to web size.

Noise of that magnitude with strong sharpening in a pixel-for-pixel crop
is to be expected.

sigh... again, he wants the "why" not the "what".

Cheers,
Alan


--
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-- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm
-- [SI gallery]: http://www.pbase.com/shootin
-- [SI rulz]: http://www.aliasimages.com/si/rulz.html
-- e-meil: there's no such thing as a FreeLunch.

"Gary Hendricks" wrote in message
om...
Hi

....

The drawback of higher ISOs are as follows:

- Larger file sizes - you'll get far fewer shots per memory card at
ISO 800 than at ISO 100.

???????

Harvey wrote:

"Gary Hendricks" wrote in message
om...

Hi


...


The drawback of higher ISOs are as follows:


- Larger file sizes - you'll get far fewer shots per memory card at
ISO 800 than at ISO 100.


???????

Hi...

Me too - all I can possibly think of is that he's
suggesting that noisy pictures don't jpeg compress
quite as much as clean ones?

Take care.

Ken

Harvey wrote:

"Gary Hendricks" wrote in message
om...

Hi


...


The drawback of higher ISOs are as follows:


- Larger file sizes - you'll get far fewer shots per memory card at
ISO 800 than at ISO 100.


???????

Hi...

Me too - all I can possibly think of is that he's
suggesting that noisy pictures don't jpeg compress
quite as much as clean ones?

Take care.

Ken

In message ,
Alan Browne wrote:

wrote:

In message ,
Alan Browne wrote:

Nah. Don't worry about assorted crankies.

No one is being cranky. I replied as I did because he may have thought
that it was an entire image shrunk to web size.

Noise of that magnitude with strong sharpening in a pixel-for-pixel crop
is to be expected.

sigh... again, he wants the "why" not the "what".

Look, I have no responsibility to choose between giving a person exactly
what they asked for, or to remain silent. There are plenty of people
here capable of answering questions. I only say something when I think
something *needs* to be said.
--


John P Sheehy