Another Digital camera question

To: All

I understand the concept of "ISO" within the context of camera film
what I don't understandis the meaning in the context of a digital
camera. What occurs when the ISO setting is changed (increased to
higher speeds) assuming that one is in Manual Mode?

Thanks.

Larry

In the film world ISO is the sensitivity of the film to light. In the
digital world it still represents the sensitivity of the whole camera to
light.

The CCD sensor essentially captures photons in each pixel and then converts
this into an integer representing the brightness of the equivalent image
pixel. Think of it like this:

image pixel brightness = (number of photons) x ISO x (some camera
constant)

The higher the ISO, the brighter the image i.e. the increased sensitivity of
the camera to light. The downside is that the higher you push the ISO, the
noisier the image becomes - this is also analogous to the world of film.
The noise is partly due to inherent weaknesses in the film or in the CCD
sensor and it is partly a limitation resulting from the physics of image
formation.

Mark

wrote in message
ups.com...
To: All

I understand the concept of "ISO" within the context of camera film
what I don't understandis the meaning in the context of a digital
camera. What occurs when the ISO setting is changed (increased to
higher speeds) assuming that one is in Manual Mode?

Thanks.

Larry

On Jul 14, 9:12 am, "Mark Shelley" wrote:
In the film world ISO is the sensitivity of the film to light. In the
digital world it still represents the sensitivity of the whole camera to
light.

The CCD sensor essentially captures photons in each pixel and then converts
this into an integer representing the brightness of the equivalent image
pixel. Think of it like this:

image pixel brightness = (number of photons) x ISO x (some camera
constant)

The higher the ISO, the brighter the image i.e. the increased sensitivity of
the camera to light. The downside is that the higher you push the ISO, the
noisier the image becomes - this is also analogous to the world of film.
The noise is partly due to inherent weaknesses in the film or in the CCD
sensor and it is partly a limitation resulting from the physics of image
formation.

Mark

wrote in message

ups.com...



To: All

I understand the concept of "ISO" within the context of camera film
what I don't understandis the meaning in the context of a digital
camera. What occurs when the ISO setting is changed (increased to
higher speeds) assuming that one is in Manual Mode?

Thanks.

Larry- Hide quoted text -

- Show quoted text -

Mark:

Thanks. Your first sentence "In the film world ISO is the sensitivity
of the film to light. In the digital world it still represents the
sensitivity of the whole camera to light." highlights my lack of
understanding. What aspect of digital camera when I increase ISO. In
Manual Mode, I set the exposure time and aperture value. What changes
when I increase the ISO setting?

Thanks.

Larry

On Sat, 14 Jul 2007 06:48:17 -0700, "
wrote:

I understand the concept of "ISO" within the context of camera film
what I don't understandis the meaning in the context of a digital
camera. What occurs when the ISO setting is changed (increased to
higher speeds) assuming that one is in Manual Mode?

Each pixel of the sensor collects photons and converts them to
electrons. During readout, the charge in each pixel (basically, the
number of electrons) is converted to a voltage, and that is then
amplified and converted to a digital value. The ISO setting determines
the amount of amplification applied.

For the most part, changing the gain (ISO setting) has very little
effect on the image data. That's because in most cameras the gain stage
is after the readout stage, and noise gets amplified to the same degree
as signal. In other words, the S/N remains the same. The actual
sensitivity of a digital camera is determined by the choice of sensor,
and not by the choice of ISO setting.

The ISO setting is useful for terrestrial imaging to help ensure that
the limited dynamic range of a JPEG file is utilized efficiently. For
most astronomical imaging (normally, dim targets), ISO settings are not
useful- especially if you have a raw mode file output. You will get the
same results using a high or low ISO setting. Actually, there's
generally one particular setting that gives the best S/N- usually one of
the lower settings, 100 or 200.

Astronomical cameras usually have their gain set a fixed value designed
to just produce a saturated output signal when a pixel is holding its
maximum number of electrons. Since that's usually about one count for
every two electrons, some cameras have the ability to change their gain
to achieve additional sensitivity when the overall source is
particularly dim. There's no point, however, to having the gain exceed
one count per electron.
_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

On Jul 14, 9:47 am, Chris L Peterson wrote:
On Sat, 14 Jul 2007 06:48:17 -0700, "

wrote:
I understand the concept of "ISO" within the context of camera film
what I don't understandis the meaning in the context of a digital
camera. What occurs when the ISO setting is changed (increased to
higher speeds) assuming that one is in Manual Mode?

Each pixel of the sensor collects photons and converts them to
electrons. During readout, the charge in each pixel (basically, the
number of electrons) is converted to a voltage, and that is then
amplified and converted to a digital value. The ISO setting determines
the amount of amplification applied.

For the most part, changing the gain (ISO setting) has very little
effect on the image data. That's because in most cameras the gain stage
is after the readout stage, and noise gets amplified to the same degree
as signal. In other words, the S/N remains the same. The actual
sensitivity of a digital camera is determined by the choice of sensor,
and not by the choice of ISO setting.

The ISO setting is useful for terrestrial imaging to help ensure that
the limited dynamic range of a JPEG file is utilized efficiently. For
most astronomical imaging (normally, dim targets), ISO settings are not
useful- especially if you have a raw mode file output. You will get the
same results using a high or low ISO setting. Actually, there's
generally one particular setting that gives the best S/N- usually one of
the lower settings, 100 or 200.

Astronomical cameras usually have their gain set a fixed value designed
to just produce a saturated output signal when a pixel is holding its
maximum number of electrons. Since that's usually about one count for
every two electrons, some cameras have the ability to change their gain
to achieve additional sensitivity when the overall source is
particularly dim. There's no point, however, to having the gain exceed
one count per electron.
_________________________________________________

Chris L Peterson
Cloudbait Observatoryhttp://www.cloudbait.com

Chris:

Thanks. This was extremely helpful.

"The ISO setting determines the amount of amplification applied. "

"For the most part, changing the gain (ISO setting) has very little
effect on the image data. That's because in most cameras the gain
stage
is after the readout stage, and noise gets amplified to the same
degree
as signal. In other words, the S/N remains the same. The actual
sensitivity of a digital camera is determined by the choice of
sensor,
and not by the choice of ISO setting. "

Thanks again.

Larry

In article .com,
wrote:

Thanks. Your first sentence "In the film world ISO is the sensitivity
of the film to light. In the digital world it still represents the
sensitivity of the whole camera to light." highlights my lack of
understanding. What aspect of digital camera when I increase ISO. In
Manual Mode, I set the exposure time and aperture value. What changes
when I increase the ISO setting?

When you increase the ISO setting, the amount by which an amplifier in
the CCD chip in the camera multiplies the signal before digitising it
is increased. So each photon hitting the camera generates a larger
fraction of a count in the output from the CCD.

Astronomical CCDs often have only the equivalent of a 'very high' ISO
setting.

Tom

On 14 Jul 2007 16:36:08 +0100 (BST), Thomas Womack
wrote:

When you increase the ISO setting, the amount by which an amplifier in
the CCD chip in the camera multiplies the signal before digitising it
is increased. So each photon hitting the camera generates a larger
fraction of a count in the output from the CCD.

Astronomical CCDs often have only the equivalent of a 'very high' ISO
setting.

Actually, astronomical cameras have the equivalent of a very low ISO
setting. They hardly apply any gain at all. This is in contrast to
digital cameras which apply a high gain when the ISO setting is large.
My Canon 300D is closest in performance to my ST8 when the former is
operating at ISO 200.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

On Jul 14, 11:16 am, Sam Wormley wrote:
wrote:
To: All

I understand the concept of "ISO" within the context of camera film
what I don't understandis the meaning in the context of a digital
camera. What occurs when the ISO setting is changed (increased to
higher speeds) assuming that one is in Manual Mode?

Thanks.

Larry

I use to fiddle with the formulae for exposure--many of the older
photography books (the Leica Manual comes to mind) gave formulae
for exposure time as a function of film sensitivity and brightness.

The general form is show athttp://en.wikipedia.org/wiki/APEX_system

I used to write

EV = Log_2 (fc·ASA/25)

where EV equals log to the base 2 of (brightness in foot candles (fc)
times the ASA speed divided by 25, which is the appropriate constant
when measuring brightness in foot candles.

Essentially ISO replaces ASA with equivalent value. Also

EV = Log_2 (aperture^2/exposure_time)

In the case of telescopic optics, the aperture is fixed, such as
f/7.5 on of the AP 160mm refractor.

Probably the most important consideration with digital astrophotography
is the sensor noise (some what analogous to film grain)... which is
minimized with lower ISO settings.

Also see:
Surface Brightness of Deep-Sky Objects Measured with a Digital Camera
http://www.clarkvision.com/astro/sur...files/introduc...

Digital Imaging Information
http://clarkvision.com/imagedetail/index.html

Sam:

Thanks for the links..

Larry

In article ,
Chris L Peterson wrote:

Actually, astronomical cameras have the equivalent of a very low ISO
setting. They hardly apply any gain at all. This is in contrast to
digital cameras which apply a high gain when the ISO setting is large.
My Canon 300D is closest in performance to my ST8 when the former is
operating at ISO 200.

I thought the whole selling-point of CCDs for astronomy was that they
produced 0.8 or so counts per input photon, and so were vastly more
sensitive than film; and that the ISO settings on digital cameras were
set so as to knock down this sensitivity to what was expected of film.
I must be missing something.

I'm not sure how full-well effects interact with ISO settings; somehow
I'd expect the level of blooming to be independent of ISO in a camera.

Tom

In article ,
Thomas Womack wrote:
In article ,
Chris L Peterson wrote:

Actually, astronomical cameras have the equivalent of a very low ISO
setting. They hardly apply any gain at all. This is in contrast to
digital cameras which apply a high gain when the ISO setting is large.
My Canon 300D is closest in performance to my ST8 when the former is
operating at ISO 200.

I thought the whole selling-point of CCDs for astronomy was that they
produced 0.8 or so counts per input photon, and so were vastly more
sensitive than film; and that the ISO settings on digital cameras were
set so as to knock down this sensitivity to what was expected of film.
I must be missing something.

ISO settings for digital cameras can go up to 2000 and beyond. I don't
ever remember having seen a traditional film with such a high ISO
rating.

I'm not sure how full-well effects interact with ISO settings; somehow
I'd expect the level of blooming to be independent of ISO in a camera.

Tom


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