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.
On Sun, 8 Jun 2008 05:46:44 -0700 (PDT), Marc Wossner
<marc.wossner@gmx.net> wrote:
>
>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.
>
> 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.
On 8 Jun., 20:23, John P Sheehy <j...@no.kom> wrote:
> Marc Wossner <marc.woss...@gmx.net> wrote in news:69f5f5bd-a9cb-46e4-
> 9d4f-d29e29d1e...@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?
Marc Wossner <marc.wossner@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.
On 8 Jun., 23:54, John P Sheehy <j...@no.kom> wrote:
> Marc Wossner <marc.woss...@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"?
On Sun, 8 Jun 2008 15:53:10 -0700 (PDT), Marc Wossner
<marc.wossner@gmx.net> wrote:
>On 8 Jun., 23:54, John P Sheehy <j...@no.kom> wrote:
>> Marc Wossner <marc.woss...@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.)
Marc Wossner <marc.wossner@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.
In article
<fe20b2a3-f6ed-4ad7-9011-caaa8b996b95@a70g2000hsh.googlegroups.com>,
Marc Wossner <marc.wossner@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 ****ed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)
>
> 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.
Bias value of Canon DSLRs?! 
Linear Mode
