Bias value of Canon DSLRs?!

Discussion in 'Digital Photography' started by Marc Wossner, Jun 8, 2008.

  1. Marc Wossner

    Marc Wossner Guest

    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.

    Best regards!
    Marc Wossner
     
    Marc Wossner, Jun 8, 2008
    #1
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  2. Marc Wossner <> wrote in news:69f5f5bd-a9cb-46e4-
    :

    >
    > 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
     
    John P Sheehy, Jun 8, 2008
    #2
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  3. Marc Wossner

    Marc Wossner Guest

    On 8 Jun., 20:23, John P Sheehy <> wrote:
    > Marc Wossner <> wrote in news:69f5f5bd-a9cb-46e4-
    > :
    >
    >
    >
    > > 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
     
    Marc Wossner, Jun 8, 2008
    #3
  4. Marc Wossner <> wrote in
    news::

    > 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
     
    John P Sheehy, Jun 8, 2008
    #4
  5. Marc Wossner

    Marc Wossner Guest

    On 8 Jun., 23:54, John P Sheehy <> wrote:
    > Marc Wossner <> wrote innews::
    >
    > > 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
     
    Marc Wossner, Jun 8, 2008
    #5
  6. Marc Wossner

    Archibald Guest

    On Sun, 8 Jun 2008 15:53:10 -0700 (PDT), Marc Wossner
    <> wrote:

    >On 8 Jun., 23:54, John P Sheehy <> wrote:
    >> Marc Wossner <> wrote innews::
    >>
    >> > 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
     
    Archibald, Jun 9, 2008
    #6
  7. Marc Wossner <> wrote in
    news::

    > 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
     
    John P Sheehy, Jun 9, 2008
    #7
  8. In article
    <>,
    Marc Wossner <> 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)
     
    Kennedy McEwen, Jun 9, 2008
    #8
  9. Kennedy McEwen <> wrote in
    news::

    > 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
     
    John P Sheehy, Jun 9, 2008
    #9
  10. In article <Xns9AB85B1361274jpsnokom@130.81.64.196>, John P Sheehy
    <> writes
    >Kennedy McEwen <> wrote in
    >news::
    >
    >> 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)
     
    Kennedy McEwen, Jun 9, 2008
    #10
  11. In article <g2j8sn$dud$>, lid 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)
     
    Kennedy McEwen, Jun 9, 2008
    #11
  12. Kennedy McEwen <> 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
     
    John P Sheehy, Jun 10, 2008
    #12
  13. Kennedy McEwen <> wrote in
    news::

    >>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
     
    John P Sheehy, Jun 10, 2008
    #13
  14. In article <Xns9AB8CBDECC463jpsnokom@130.81.64.196>, John P Sheehy
    <> writes
    >Kennedy McEwen <> wrote in
    >news::
    >
    >>>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".
    >

    Which, in the case of the Canon cameras is not the case for the majority
    of the exposure range.

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

    Quite, you should try it sometime!
    --
    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)
     
    Kennedy McEwen, Jun 10, 2008
    #14
  15. In article <Xns9AB8CB6FAA29Djpsnokom@130.81.64.196>, John P Sheehy
    <> writes
    >Kennedy McEwen <> 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.


    I don't believe it has to "decide" anything of the sort.

    > The 10D was
    >inconsistent, the newer cameras are not,


    You're sure about that?

    > 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.
    >

    For many of the reasons given already, a factory calibration would be
    likely to achieve anywhere close to the precision necessary - 1 part in
    4096. Analogue electronics just isn't that stable.

    A self test would be "observable" to the user, especially at long
    exposures.

    Unexposed pixels might be possible, but again, I doubt this would
    achieve the expected consistency. Unexposed pixels are outside the edge
    of the frame - a look at the black level of pixels inside the edge of
    the frame, especially at long exposures, shows a significantly different
    mean black level from the central area.
    --
    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)
     
    Kennedy McEwen, Jun 10, 2008
    #15
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