Bias value of Canon DSLRs?!

On 8 Jun., 20:23, John P Sheehy <j... RemoveThis @no.kom> wrote:
> Marc Wossner <marc.woss... RemoveThis @gmx.net> wrote in news:69f5f5bd-a9cb-46e4-
> 9d4f-d29e29d1e... RemoveThis @m73g2000hsh.googlegroups.com:
>
>
>
> > Hi ng,
>
> > does anyone know how large the bias or offset is that Canon applies to
> > its DSLRs in order not to clipp negative voltage fluctuations to zero?
> > I read it must be subtracted from the average raw value of an image in
> > the course of analysing the signal to noise ratio.
>
> It can vary a little bit, especially with long exposures at high ISOs,
> but generally, the 14-bit cameras use 1024, the non-rebels and the 300D
> use 128, and the other rebels (and also the 10D at ISOs 1600 and 3200 use
> 256).  I don't recall what the 1Ds cameras do at ISO 3200, but the mk1
> and/or mk2 may use 256 instead of 128.
>
> --
> John Sheehy

Interesting, no fixed value. Does Canon state somewhere what bias they
use in what camera (I havent found any reference on their website) or
can it be calculated from some raw values?

Best regards!
Marc Wossner
 >> Stay informed about: Bias value of Canon DSLRs?! 

On 8 Jun., 23:54, John P Sheehy <j... DeleteThis @no.kom> wrote:
> Marc Wossner <marc.woss... DeleteThis @gmx.net> wrote innews:fe20b2a3-f6ed-4ad7-9011-caaa8b996b95@a70g2000hsh.googlegroups.com:
>
> > Interesting, no fixed value. Does Canon state somewhere what bias they
> > use in what camera (I havent found any reference on their website) or
> > can it be calculated from some raw values?
>
> Just take a black 'exposure" and see what the average is.
>
> Canon and most every other company thinks we're a bunch of morons with no
> right to any knowledge of how our cameras work under the hood.
>
> --
> John Sheehy


Sorry, Im not so deep into that.
I guess a black exposure is a dark frame with short exposure time and
the lens cap on but what do you mean by "the average"?

Best regards!
Marc Wossner
 >> Stay informed about: Bias value of Canon DSLRs?! 

Marc Wossner <marc.wossner DeleteThis @gmx.net> wrote in news:69f5f5bd-a9cb-46e4-
9d4f-d29e29d1e588 DeleteThis @m73g2000hsh.googlegroups.com:

>
> Hi ng,
>
> does anyone know how large the bias or offset is that Canon applies to
> its DSLRs in order not to clipp negative voltage fluctuations to zero?
> I read it must be subtracted from the average raw value of an image in
> the course of analysing the signal to noise ratio.

It can vary a little bit, especially with long exposures at high ISOs,
but generally, the 14-bit cameras use 1024, the non-rebels and the 300D
use 128, and the other rebels (and also the 10D at ISOs 1600 and 3200 use
256). I don't recall what the 1Ds cameras do at ISO 3200, but the mk1
and/or mk2 may use 256 instead of 128.



--
John Sheehy
 >> Stay informed about: Bias value of Canon DSLRs?! 

Marc Wossner <marc.wossner DeleteThis @gmx.net> wrote in
news:fe20b2a3-f6ed-4ad7-9011-caaa8b996b95@a70g2000hsh.googlegroups.com:

> Interesting, no fixed value. Does Canon state somewhere what bias they
> use in what camera (I havent found any reference on their website) or
> can it be calculated from some raw values?

Just take a black 'exposure" and see what the average is.

Canon and most every other company thinks we're a bunch of morons with no
right to any knowledge of how our cameras work under the hood.



--
John Sheehy
 >> Stay informed about: Bias value of Canon DSLRs?! 

On Sun, 8 Jun 2008 15:53:10 -0700 (PDT), Marc Wossner
<marc.wossner RemoveThis @gmx.net> wrote:

>On 8 Jun., 23:54, John P Sheehy <j... RemoveThis @no.kom> wrote:
>> Marc Wossner <marc.woss... RemoveThis @gmx.net> wrote innews:fe20b2a3-f6ed-4ad7-9011-caaa8b996b95@a70g2000hsh.googlegroups.com:
>>
>> > Interesting, no fixed value. Does Canon state somewhere what bias they
>> > use in what camera (I havent found any reference on their website) or
>> > can it be calculated from some raw values?
>>
>> Just take a black 'exposure" and see what the average is.
>>
>> Canon and most every other company thinks we're a bunch of morons with no
>> right to any knowledge of how our cameras work under the hood.
>>
>> --
>> John Sheehy
>
>
>Sorry, Im not so deep into that.
>I guess a black exposure is a dark frame with short exposure time and
>the lens cap on but what do you mean by "the average"?

I guess you were just called a moron. (I'm one too.)

Archibald
 >> Stay informed about: Bias value of Canon DSLRs?! 

Marc Wossner <marc.wossner.DeleteThis@gmx.net> wrote in
news:7726b35a-0d7f-4984-98e6-efb917b9809c@y38g2000hsy.googlegroups.com:

> Sorry, Im not so deep into that.
> I guess a black exposure is a dark frame with short exposure time and
> the lens cap on but what do you mean by "the average"?

The average RAW value of all the unexposed pixels in the black frame.

--
John Sheehy
 >> Stay informed about: Bias value of Canon DSLRs?! 

In article
<fe20b2a3-f6ed-4ad7-9011-caaa8b996b95.TakeThisOut@a70g2000hsh.googlegroups.com>,
Marc Wossner <marc.wossner.TakeThisOut@gmx.net> writes
>
>Interesting, no fixed value. Does Canon state somewhere what bias they
>use in what camera (I havent found any reference on their website) or
>can it be calculated from some raw values?
>
Marc,
how do you expect a fixed value to be achieved in practice?

Remember this is an ANALOGUE offset. For example, if the "target"
offset is 128 on a 12 bit ADC then the total offset is only 3% of the
entire analogue range and needs adjustment with a precision of only
0.25%. Analogue signals are simply not that precise or consistent and
will vary by far more than this with age and over the operating
temperature range of the unit.
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's pissed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)
 >> Stay informed about: Bias value of Canon DSLRs?! 

Kennedy McEwen wrote:

>
> Remember this is an ANALOGUE offset. For example, if the "target"
> offset is 128 on a 12 bit ADC then the total offset is only 3% of the
> entire analogue range and needs adjustment with a precision of only
> 0.25%. Analogue signals are simply not that precise or consistent and
> will vary by far more than this with age and over the operating
> temperature range of the unit.


Uh ... a good ADC should be accurate and stable to 11 or 12 bits with no problem,
even with age or temperature. 16 bits is a different matter, at that
precision only the best will be that accurate or stable over long times.

OF course, the sensor pixels themselves will have a zero offset that varies with temperature
simply due to the properties of silicon, unless they have installed some
sort of temperature compensation. Most cameras as far as I know get that
compensation with a dark frame, if desired.

Doug McDonald
 >> Stay informed about: Bias value of Canon DSLRs?! 

Kennedy McEwen <rkm DeleteThis @nospam.demon.co.uk> wrote in
news:URSbdRBZFNTIFwxE@kennedym.demon.co.uk:

> Remember this is an ANALOGUE offset. For example, if the "target"
> offset is 128 on a 12 bit ADC then the total offset is only 3% of the
> entire analogue range and needs adjustment with a precision of only
> 0.25%. Analogue signals are simply not that precise or consistent and
> will vary by far more than this with age and over the operating
> temperature range of the unit.

With the newer Canons, it seems to be calculated on the fly. The 10D
"guessed" the exact offset, and was often a bit off.


--
John Sheehy
 >> Stay informed about: Bias value of Canon DSLRs?! 

In article <Xns9AB85B1361274jpsnokom.DeleteThis@130.81.64.196>, John P Sheehy
<jps.DeleteThis@no.kom> writes
>Kennedy McEwen <rkm.DeleteThis@nospam.demon.co.uk> wrote in
>news:URSbdRBZFNTIFwxE@kennedym.demon.co.uk:
>
>> Remember this is an ANALOGUE offset. For example, if the "target"
>> offset is 128 on a 12 bit ADC then the total offset is only 3% of the
>> entire analogue range and needs adjustment with a precision of only
>> 0.25%. Analogue signals are simply not that precise or consistent and
>> will vary by far more than this with age and over the operating
>> temperature range of the unit.
>
>With the newer Canons, it seems to be calculated on the fly. The 10D
>"guessed" the exact offset, and was often a bit off.
>
Since it is analogue it can't be "calculated", whether on the fly or
otherwise. What is being offset is the analogue voltage input to the
ADC, the digital output is only the consequence of that offset.
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's pissed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)
 >> Stay informed about: Bias value of Canon DSLRs?! 

In article <g2j8sn$dud$1@news.acm.uiuc.edu>, user RemoveThis @domain.invalid writes
>Kennedy McEwen wrote:
>
>> Remember this is an ANALOGUE offset. For example, if the "target"
>>offset is 128 on a 12 bit ADC then the total offset is only 3% of the
>>entire analogue range and needs adjustment with a precision of only
>>0.25%. Analogue signals are simply not that precise or consistent and
>>will vary by far more than this with age and over the operating
>>temperature range of the unit.
>
>
>Uh ... a good ADC should be accurate and stable to 11 or 12 bits with
>no problem,
>even with age or temperature. 16 bits is a different matter, at that
>precision only the best will be that accurate or stable over long times.
>
The ADC itself may be exceedingly accurate in the levels that its zero
and full scale are set, but that isn't the problem - it is offsetting
the analogue input by a fixed and consistent amount, not only over
temperature but over millions of different circuits for their entire
life.

>OF course, the sensor pixels themselves will have a zero offset that
>varies with temperature
>simply due to the properties of silicon, unless they have installed some
>sort of temperature compensation. Most cameras as far as I know get that
>compensation with a dark frame, if desired.
>
The only time a "dark frame" is used with the Canon cameras is when long
exposure noise reduction is enabled in CF-02.
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's pissed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)
 >> Stay informed about: Bias value of Canon DSLRs?! 

Kennedy McEwen <rkm.DeleteThis@nospam.demon.co.uk> wrote in news:C5yeX9AaIVTIFwQ1
@kennedym.demon.co.uk:

> Since it is analogue it can't be "calculated", whether on the fly or
> otherwise. What is being offset is the analogue voltage input to the
> ADC, the digital output is only the consequence of that offset.

The camera still has to decide what that offset will be, to get the analog
zero signal mean to correspond to the target digitized value. The 10D was
inconsistent, the newer cameras are not, so the newer cameras are probably
looking at some unexposed pixels, or doing something else like a self-test
or factory calibration to get the desired results.

--
John Sheehy
 >> Stay informed about: Bias value of Canon DSLRs?! 

Kennedy McEwen <rkm.RemoveThis@nospam.demon.co.uk> wrote in
news:ZpCYTRBbIVTIFwTG@kennedym.demon.co.uk:

>>Most cameras as far as I know get
>>that
>>compensation with a dark frame, if desired.

> The only time a "dark frame" is used with the Canon cameras is when long
> exposure noise reduction is enabled in CF-02.

Hence, his "if desired".

You're never too smart to slow down and read!

--
John Sheehy
 >> Stay informed about: Bias value of Canon DSLRs?!