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Date: 14 Jul 2007 06:48:17
From: actuary@mchsi.com
Subject: 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





 
Date: 14 Jul 2007 19:43:08
From: William Hamblen
Subject: Re: Another Digital camera question
On Sat, 14 Jul 2007 06:48:17 -0700, "actuary@mchsi.com"
<actuary@mchsi.com > 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?

When a photon strikes a pixel in the camera it kicks off an electron
that collects in the charge well of the image chip. More electrons
mean more voltage on a given pixel. When the the image on the chip is
read out the voltage created by the charge is amplified and sent to an
analog to digital converter that converts the voltage to a number.
You are working with fairly small signals. When you change the ISO
setting on the camera you change the amount of amplification. Higher
ISO settings mean more amplification. You are also amplifying the
noise in the circuit, so the resulting image appears more noisy or
grainy with a high ISO setting than with a low. You need to
experiment with using a digital camera for long exposure
astrophotography to get the settings that work best in a given
situation.

Bud
--
The night is just the shadow of the Earth.


 
Date: 14 Jul 2007 10:37:48
From: actuary@mchsi.com
Subject: Re: Another Digital camera question
On Jul 14, 11:16 am, Sam Wormley <sworml...@mchsi.com > wrote:
> actu...@mchsi.com 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 =3D Log_2 (fc=B7ASA/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 =3D 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/surface-brightness-profiles/introdu=
c=2E..
>
> Digital Imaging Information
> http://clarkvision.com/imagedetail/index.html

Sam:

Thanks for the links..

Larry



 
Date: 14 Jul 2007 16:16:20
From: Sam Wormley
Subject: Re: Another Digital camera question
actuary@mchsi.com 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 at http://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/surface-brightness-profiles/introduction.html

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











 
Date: 14 Jul 2007 07:54:12
From: actuary@mchsi.com
Subject: Re: Another Digital camera question
On Jul 14, 9:47 am, Chris L Peterson <c...@alumni.caltech.edu > wrote:
> On Sat, 14 Jul 2007 06:48:17 -0700, "actu...@mchsi.com"
>
> <actu...@mchsi.com> 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





 
Date: 14 Jul 2007 14:47:31
From: Chris L Peterson
Subject: Re: Another Digital camera question
On Sat, 14 Jul 2007 06:48:17 -0700, "actuary@mchsi.com"
<actuary@mchsi.com > 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


 
Date: 14 Jul 2007 07:18:55
From: actuary@mchsi.com
Subject: Re: Another Digital camera question
On Jul 14, 9:12 am, "Mark Shelley" <news...@shelleyz.co.uk > 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
>
> <actu...@mchsi.com> wrote in message
>
> news:1184420897.353426.277330@57g2000hsv.googlegroups.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




  
Date: 14 Jul 2007 16:36:08
From: Thomas Womack
Subject: Re: Another Digital camera question
In article <1184422735.915610.255280@k79g2000hse.googlegroups.com >,
actuary@mchsi.com <actuary@mchsi.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


   
Date: 14 Jul 2007 15:51:44
From: Chris L Peterson
Subject: Re: Another Digital camera question
On 14 Jul 2007 16:36:08 +0100 (BST), Thomas Womack
<twomack@chiark.greenend.org.uk > 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


    
Date: 14 Jul 2007 19:47:38
From: William Hamblen
Subject: Re: Another Digital camera question
On Sat, 14 Jul 2007 15:51:44 GMT, Chris L Peterson
<clp@alumni.caltech.edu > 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.

The big advantage the better astronomical cameras have over digital
cameras is that the astronomical camera is cooled to a low temperature
to reduce the thermal noise or dark current added to the signal. You
can take longer exposures with a cooled camera before the thermal
noise gets too bad.

Bud
--
The night is just the shadow of the Earth.


     
Date: 15 Jul 2007 01:49:17
From: Chris L Peterson
Subject: Re: Another Digital camera question
On Sat, 14 Jul 2007 19:47:38 -0500, William Hamblen
<wrhamblen@comcast.net > wrote:

>The big advantage the better astronomical cameras have over digital
>cameras is that the astronomical camera is cooled to a low temperature
>to reduce the thermal noise or dark current added to the signal. You
>can take longer exposures with a cooled camera before the thermal
>noise gets too bad.

And the reason that's an advantage is because for short exposures,
readout noise is dominant- whether you have a digital camera or the best
astronomical camera. If there were no readout noise, you could build
your image out of many short exposures with no penalty at all (but you'd
still have problems with dark current noise, so you'd still need a
cooled camera).

_________________________________________________

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


    
Date: 14 Jul 2007 20:43:04
From: Thomas Womack
Subject: Re: Another Digital camera question
In article <41sh93h36rppdqjs0240242ste30hugfre@4ax.com >,
Chris L Peterson <clp@alumni.caltech.edu > 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


     
Date: 15 Jul 2007 00:51:03
From: Chris L Peterson
Subject: Re: Another Digital camera question
On 14 Jul 2007 20:43:04 +0100 (BST), Thomas Womack
<twomack@chiark.greenend.org.uk > wrote:

>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.

Digital cameras run internally at something around .2 counts per photon.
When you are shooting a dark scene at a reasonable handheld exposure,
there aren't many photons. In order to get a reasonable count, the gain
is pushed up to several counts per photon. But that takes the noise
right along with it.

With astronomical cameras, you don't usually think in terms of gain, but
in terms of photon count, which has nothing to do with any internal gain
factors that may be applied to optimize the measurement process.


>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.

Most digicams show little or no blooming because of the design of their
sensors. You just get a saturated pixel value. At higher ISO settings,
that saturation may occur well before the well gets filled up.

_________________________________________________

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


     
Date: 14 Jul 2007 21:12:34
From: Paul Schlyter
Subject: Re: Another Digital camera question
In article <KsF*sNKPr@news.chiark.greenend.org.uk >,
Thomas Womack <twomack@chiark.greenend.org.uk > wrote:
>In article <41sh93h36rppdqjs0240242ste30hugfre@4ax.com>,
>Chris L Peterson <clp@alumni.caltech.edu> 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


--
----------------------------------------------------------------
Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stockholm dot bostream dot se
WWW: http://stjarnhimlen.se/


      
Date: 14 Jul 2007 19:59:05
From: William Hamblen
Subject: Re: Another Digital camera question
On Sat, 14 Jul 2007 21:12:34 GMT, pausch@saaf.se (Paul Schlyter)
wrote:

>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.

B&H Photo advertises Kodak TMAX P3200 and Ilford Delta 3200.

Bud
--
The night is just the shadow of the Earth.


 
Date: 14 Jul 2007 14:12:11
From: Mark Shelley
Subject: Re: Another Digital camera question

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

<actuary@mchsi.com > wrote in message
news:1184420897.353426.277330@57g2000hsv.googlegroups.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
>