DSLR questions?

Discussion in 'Digital Photography' started by PeteD, Sep 26, 2006.

  1. PeteD

    PeteD Guest

    Hi,

    I recently bought my first DSLR (D80) and am getting to grips with
    differences from film and my digital P&S.

    1. I was reading about hot pixels and maybe I shouldn't have. Being an
    engineer I went and took a shot of a blank wall at ISO 800 and 1/2 sec.
    At 100% I could see a small + sign at one point and a couple of other
    coloured pixels. No problem at 1/30 sec. Is this normal? Most of my
    photography will be at 1/60 sec and faster. I don't want to get carried
    away looking for trouble!!

    2. Also spotted two dust spots when doing this as I had a small
    aperture. Further testing on sky shows the dust spots visible down to
    around f14. I know this is par for the course but what do most people
    do. Ignore it and post process or try and keep aperture wider?? I'm not
    really up for having a go at sensor cleaning two weeks into ownership!!


    This leads me to a comment I saw on Ken Rockwell's site.

    3. Article title:
    "How to Select the Sharpest Aperture Considering the Simultaneous
    Effects of Depth-of-Field and Diffraction"

    http://www.kenrockwell.com/tech/focus.htm

    The following statement puzzles me?

    "If you are a beginner or just shooting a 35mm or digital camera then
    this article addresses issues which won't bother you at reasonable
    apertures. Just use a tripod and choose the smallest aperture you have
    if you need depth of field. Avoid apertures smaller than f/8 or f/11 on
    digital cameras. "

    Why the aperture constraint?? Surely for good depth of field in a
    landscape shot you would stop it down further.

    Apologies if this all beneath you guys, I'm just trying to enjoy moving
    to digital without getting stressed over a few issues.

    Thanks
    Pete
     
    PeteD, Sep 26, 2006
    #1
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  2. PeteD

    frederick Guest

    PeteD wrote:
    > Hi,
    >
    > I recently bought my first DSLR (D80) and am getting to grips with
    > differences from film and my digital P&S.
    >
    > 1. I was reading about hot pixels and maybe I shouldn't have. Being an
    > engineer I went and took a shot of a blank wall at ISO 800 and 1/2 sec.
    > At 100% I could see a small + sign at one point and a couple of other
    > coloured pixels. No problem at 1/30 sec. Is this normal? Most of my
    > photography will be at 1/60 sec and faster. I don't want to get carried
    > away looking for trouble!!
    >

    Normal - does the D80 have long exposure noise reduction (dark frame
    subtraction)? If so, then use it for long exposures.
    >
    > 2. Also spotted two dust spots when doing this as I had a small
    > aperture. Further testing on sky shows the dust spots visible down to
    > around f14. I know this is par for the course but what do most people
    > do. Ignore it and post process or try and keep aperture wider?? I'm not
    > really up for having a go at sensor cleaning two weeks into ownership!!
    >
    >
    > This leads me to a comment I saw on Ken Rockwell's site.
    >
    > 3. Article title:
    > "How to Select the Sharpest Aperture Considering the Simultaneous
    > Effects of Depth-of-Field and Diffraction"
    >
    > http://www.kenrockwell.com/tech/focus.htm
    >
    > The following statement puzzles me?
    >
    > "If you are a beginner or just shooting a 35mm or digital camera then
    > this article addresses issues which won't bother you at reasonable
    > apertures. Just use a tripod and choose the smallest aperture you have
    > if you need depth of field. Avoid apertures smaller than f/8 or f/11 on
    > digital cameras. "
    >
    > Why the aperture constraint?? Surely for good depth of field in a
    > landscape shot you would stop it down further.
    >
    > Apologies if this all beneath you guys, I'm just trying to enjoy moving
    > to digital without getting stressed over a few issues.
    >

    Diffraction is explained well here:
    http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm#
    Ken may not have intended it, but diffraction effect isn't limited to
    digital - perhaps he meant "digital slr with cropped sensor" not
    "digital". Of course you can still use smaller apertures - but will
    lose resolving power to diffraction. That might be justified depending
    on what you are trying to achieve. For landscape where you probably
    want maximum detail retained as well as depth of field, then you
    probably want to avoid visible diffraction losses, and may benefit from
    focusing manually and calculating parfocal distance. That is further
    explained here:
    http://www.cambridgeincolour.com/tutorials/DOF-calculator.htm
     
    frederick, Sep 26, 2006
    #2
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  3. PeteD

    PeteD Guest

    Thanks for that,

    The D80 has long shutter NR for long exposures from around 8secs... it
    also has High ISO NR which is normally off for ISO's up to 800. I'll
    have a play....I read something about the + meaning it's probably a bad
    photosite or something.

    Thanks for the pointers to the articles too.

    Pete

    frederick wrote:
    > PeteD wrote:
    > > Hi,
    > >
    > > I recently bought my first DSLR (D80) and am getting to grips with
    > > differences from film and my digital P&S.
    > >
    > > 1. I was reading about hot pixels and maybe I shouldn't have. Being an
    > > engineer I went and took a shot of a blank wall at ISO 800 and 1/2 sec.
    > > At 100% I could see a small + sign at one point and a couple of other
    > > coloured pixels. No problem at 1/30 sec. Is this normal? Most of my
    > > photography will be at 1/60 sec and faster. I don't want to get carried
    > > away looking for trouble!!
    > >

    > Normal - does the D80 have long exposure noise reduction (dark frame
    > subtraction)? If so, then use it for long exposures.
    > >
    > > 2. Also spotted two dust spots when doing this as I had a small
    > > aperture. Further testing on sky shows the dust spots visible down to
    > > around f14. I know this is par for the course but what do most people
    > > do. Ignore it and post process or try and keep aperture wider?? I'm not
    > > really up for having a go at sensor cleaning two weeks into ownership!!
    > >
    > >
    > > This leads me to a comment I saw on Ken Rockwell's site.
    > >
    > > 3. Article title:
    > > "How to Select the Sharpest Aperture Considering the Simultaneous
    > > Effects of Depth-of-Field and Diffraction"
    > >
    > > http://www.kenrockwell.com/tech/focus.htm
    > >
    > > The following statement puzzles me?
    > >
    > > "If you are a beginner or just shooting a 35mm or digital camera then
    > > this article addresses issues which won't bother you at reasonable
    > > apertures. Just use a tripod and choose the smallest aperture you have
    > > if you need depth of field. Avoid apertures smaller than f/8 or f/11 on
    > > digital cameras. "
    > >
    > > Why the aperture constraint?? Surely for good depth of field in a
    > > landscape shot you would stop it down further.
    > >
    > > Apologies if this all beneath you guys, I'm just trying to enjoy moving
    > > to digital without getting stressed over a few issues.
    > >

    > Diffraction is explained well here:
    > http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm#
    > Ken may not have intended it, but diffraction effect isn't limited to
    > digital - perhaps he meant "digital slr with cropped sensor" not
    > "digital". Of course you can still use smaller apertures - but will
    > lose resolving power to diffraction. That might be justified depending
    > on what you are trying to achieve. For landscape where you probably
    > want maximum detail retained as well as depth of field, then you
    > probably want to avoid visible diffraction losses, and may benefit from
    > focusing manually and calculating parfocal distance. That is further
    > explained here:
    > http://www.cambridgeincolour.com/tutorials/DOF-calculator.htm
     
    PeteD, Sep 26, 2006
    #3
  4. PeteD

    frederick Guest

    PeteD wrote:
    > Thanks for that,
    >
    > The D80 has long shutter NR for long exposures from around 8secs... it
    > also has High ISO NR which is normally off for ISO's up to 800. I'll
    > have a play....I read something about the + meaning it's probably a bad
    > photosite or something.
    >
    > Thanks for the pointers to the articles too.
    >
    > Pete
    >

    Are you sure it's 8 seconds?
    It just strikes me that you would probably expect to see such noise at
    exposures longer than 1 second or so.
    If it is disabled when exposure is shorter, then that is good. The D70
    didn't do that - so when long exposure NR was left on, users wondered
    why the burst frame rate was well "below spec".
     
    frederick, Sep 26, 2006
    #4
  5. PeteD

    acl Guest

    frederick wrote:

    > Are you sure it's 8 seconds?
    > It just strikes me that you would probably expect to see such noise at
    > exposures longer than 1 second or so.


    On the D200 at least (and presumably also on the D80) I have found that
    it makes no (positive) difference up to around ISO 640 for exposures up
    to 30s. I have not tested longer exposures systematically. For ISO 800
    and around 30s, it's slightly better to have dark frame subtraction on.
    I have tested exposures of several minutes at various ISOs out of
    boredom, and at eg ISO 400 and 6 min it's better to have it on (removes
    purple blobs due to heat from electronics). It's just a matter of
    testing your camera.

    I may remember some of these numbers incorrectly; anyway, testing is
    for free!

    > If it is disabled when exposure is shorter, then that is good. The D70
    > didn't do that - so when long exposure NR was left on, users wondered
    > why the burst frame rate was well "below spec".


    Well, if dark frame subtraction is on, the D200 shows a smaller number
    of available exposures before the buffer fills, although DFS does not
    occur unless the exposure is over 7s (or thereabouts). I have no idea
    if it actually does stop taking pictures if the counter reaches zero
    with DFS on, but I doubt it.
     
    acl, Sep 26, 2006
    #5
  6. PeteD

    frederick Guest

    acl wrote:
    > frederick wrote:
    >
    >> Are you sure it's 8 seconds?
    >> It just strikes me that you would probably expect to see such noise at
    >> exposures longer than 1 second or so.

    >
    > On the D200 at least (and presumably also on the D80) I have found that
    > it makes no (positive) difference up to around ISO 640 for exposures up
    > to 30s. I have not tested longer exposures systematically. For ISO 800
    > and around 30s, it's slightly better to have dark frame subtraction on.
    > I have tested exposures of several minutes at various ISOs out of
    > boredom, and at eg ISO 400 and 6 min it's better to have it on (removes
    > purple blobs due to heat from electronics). It's just a matter of
    > testing your camera.
    >
    > I may remember some of these numbers incorrectly; anyway, testing is
    > for free!
    >

    I have since checked specs - it appears to be automatically "off" at
    exposures shorter than 1/2 second on the D80. As well as correcting for
    the "purple blobs" from heat, it should correct for hot pixels. Perhaps
    if you shoot at lowest ISO you will see an advantage with exposures
    shorter than 30 seconds.
     
    frederick, Sep 26, 2006
    #6
  7. PeteD

    acl Guest

    frederick wrote:
    > I have since checked specs - it appears to be automatically "off" at
    > exposures shorter than 1/2 second on the D80. As well as correcting for
    > the "purple blobs" from heat, it should correct for hot pixels. Perhaps
    > if you shoot at lowest ISO you will see an advantage with exposures
    > shorter than 30 seconds.


    No, the higher the ISO, the shorter the exposure that necessitates use
    of DFS. Not surprisingly: the idea is that the DFS adds noise to the
    image (the random part that is not common to the two frames). So it's
    only useful if there are systematic noise sources (to wit, the purple
    blob and hot pixels mentioned, or some kind of banding etc). These
    appear at longer exposures and higher ISOs, for obvious reasons.
     
    acl, Sep 26, 2006
    #7
  8. PeteD

    frederick Guest

    acl wrote:
    > frederick wrote:
    >> I have since checked specs - it appears to be automatically "off" at
    >> exposures shorter than 1/2 second on the D80. As well as correcting
    >> for the "purple blobs" from heat, it should correct for hot pixels.
    >> Perhaps if you shoot at lowest ISO you will see an advantage with
    >> exposures shorter than 30 seconds.

    >
    > No, the higher the ISO, the shorter the exposure that necessitates use
    > of DFS. Not surprisingly: the idea is that the DFS adds noise to the
    > image (the random part that is not common to the two frames). So it's
    > only useful if there are systematic noise sources (to wit, the purple
    > blob and hot pixels mentioned, or some kind of banding etc). These
    > appear at longer exposures and higher ISOs, for obvious reasons.


    I would have thought that as DFS removed noise that was repeatable
    patterns from hot pixels etc, then it would be rendered more useless as
    ISO increased random noise patterns which obscured those repeatable
    patterns. Then again I got that idea from observing results.
     
    frederick, Sep 26, 2006
    #8
  9. PeteD

    acl Guest

    frederick wrote:
    > acl wrote:
    > > frederick wrote:
    > >> I have since checked specs - it appears to be automatically "off" at
    > >> exposures shorter than 1/2 second on the D80. As well as correcting
    > >> for the "purple blobs" from heat, it should correct for hot pixels.
    > >> Perhaps if you shoot at lowest ISO you will see an advantage with
    > >> exposures shorter than 30 seconds.

    > >
    > > No, the higher the ISO, the shorter the exposure that necessitates use
    > > of DFS. Not surprisingly: the idea is that the DFS adds noise to the
    > > image (the random part that is not common to the two frames). So it's
    > > only useful if there are systematic noise sources (to wit, the purple
    > > blob and hot pixels mentioned, or some kind of banding etc). These
    > > appear at longer exposures and higher ISOs, for obvious reasons.

    >
    > I would have thought that as DFS removed noise that was repeatable
    > patterns from hot pixels etc, then it would be rendered more useless as
    > ISO increased random noise patterns which obscured those repeatable
    > patterns. Then again I got that idea from observing results.


    It seems we're saying the same thing. I suppose my point was that these
    repeatable patterns (hot pixels, mainly) are more likely to appear at a
    given exposure time if the ISO is higher. Yes, random noise (not
    removed by DFS, as you say) will increase, but hot pixels will also
    increase. I have never seen hot pixels at up to 30s on ISO 100 and 200
    on my camera, for example.

    Anyway, I noticed that up to around ISO 400 or so (maybe a bit more),
    and up to 30s, switching DFS on actually degrades the image on my
    camera (it has more random noise; mind you, the difference is small).
    So I thought a bit about it and realised that since both frames have
    random noise, and you subtract them, the result will have even more
    random noise (assuming it is zero-average, addition and subtraction are
    the same, for our purposes: the noise is as likely to be up as down;
    ignoring some details). Not really enough to worry about, but the extra
    time is irritating.
     
    acl, Sep 27, 2006
    #9
  10. PeteD

    Paul Rubin Guest

    "acl" <> writes:
    > Anyway, I noticed that up to around ISO 400 or so (maybe a bit more),
    > and up to 30s, switching DFS on actually degrades the image on my
    > camera (it has more random noise; mind you, the difference is small).
    > So I thought a bit about it and realised that since both frames have
    > random noise, and you subtract them, the result will have even more
    > random noise (assuming it is zero-average, ...


    But in this case the camera could compensate by averaging together a
    bunch of dark frames. The random components would cancel out and the
    systematic components would stay.
     
    Paul Rubin, Sep 27, 2006
    #10
  11. PeteD

    ASAAR Guest

    On 26 Sep 2006 17:19:08 -0700, acl wrote:

    > Anyway, I noticed that up to around ISO 400 or so (maybe a bit more),
    > and up to 30s, switching DFS on actually degrades the image on my
    > camera (it has more random noise; mind you, the difference is small).
    > So I thought a bit about it and realised that since both frames have
    > random noise, and you subtract them, the result will have even more
    > random noise (assuming it is zero-average, addition and subtraction are
    > the same, for our purposes: the noise is as likely to be up as down;
    > ignoring some details). Not really enough to worry about, but the extra
    > time is irritating.


    Though unlikely to be implemented, it might be possible to design
    a camera such that most of the DFS benefit is obtained even when it
    is disabled. It would require enough memory to hold a small number
    of Dark Frame Profiles, maybe a one or two dozen, created by having
    the camera store what it has learned by creating Dark Frames over
    its range of shutter speeds and over a smaller range of
    temperatures. Every possible combination wouldn't be needed, as the
    camera could use interpolation to save time and space. But truly
    fussy photographers should have the ability to use an actual Dark
    Frame at any time, bypassing the DFPs if they wish. A default
    profile could be created for each camera by the manufacturer as part
    of the burn-in process, and this could also be available as a menu
    option to allow the DFPs to be refreshed as the camera ages.

    One immediate benefit, even if all profiles were deleted, is that
    if you took a series of pictures in one setting where all of the
    exposures were the same, the result of the first DF would apply to
    each subsequent shot, saving the extra time as long as the exposure
    remains unchanged. There could be a flag displayed in the
    viewfinder when DFS is enabled, indicating if at the chosen
    exposure, the camera would actually have to take the time needed to
    create a Dark Frame. Another benefit if dual Dark Frames are
    created is that since as you said, random noise would differ in each
    DF but the effect of hot pixels wouldn't, by comparing the two,
    practically the only thing the camera would subtract from the actual
    image would be hot pixels, not the random contribution from noise.
     
    ASAAR, Sep 27, 2006
    #11
  12. PeteD

    acl Guest

    Paul Rubin wrote:
    > "acl" <> writes:
    > > Anyway, I noticed that up to around ISO 400 or so (maybe a bit more),
    > > and up to 30s, switching DFS on actually degrades the image on my
    > > camera (it has more random noise; mind you, the difference is small).
    > > So I thought a bit about it and realised that since both frames have
    > > random noise, and you subtract them, the result will have even more
    > > random noise (assuming it is zero-average, ...

    >
    > But in this case the camera could compensate by averaging together a
    > bunch of dark frames. The random components would cancel out and the
    > systematic components would stay.


    Right, but it doesn't, it only takes one. Anyway, in practice the
    difference is not much between DFS on and off (as far as random noise
    is concerned).
     
    acl, Sep 27, 2006
    #12
  13. PeteD

    acl Guest

    ASAAR wrote:
    > Though unlikely to be implemented, it might be possible to design
    > a camera such that most of the DFS benefit is obtained even when it
    > is disabled. It would require enough memory to hold a small number
    > of Dark Frame Profiles, maybe a one or two dozen, created by having
    > the camera store what it has learned by creating Dark Frames over
    > its range of shutter speeds and over a smaller range of
    > temperatures. Every possible combination wouldn't be needed, as the
    > camera could use interpolation to save time and space. But truly
    > fussy photographers should have the ability to use an actual Dark
    > Frame at any time, bypassing the DFPs if they wish. A default
    > profile could be created for each camera by the manufacturer as part
    > of the burn-in process, and this could also be available as a menu
    > option to allow the DFPs to be refreshed as the camera ages.


    Yes, and the same could be achieved by doing it manually (taking a dark
    frame and then subtracting it in eg photoshop). It's not as convenient,
    though.

    >
    > One immediate benefit, even if all profiles were deleted, is that
    > if you took a series of pictures in one setting where all of the
    > exposures were the same, the result of the first DF would apply to
    > each subsequent shot, saving the extra time as long as the exposure
    > remains unchanged. There could be a flag displayed in the
    > viewfinder when DFS is enabled, indicating if at the chosen
    > exposure, the camera would actually have to take the time needed to
    > create a Dark Frame. Another benefit if dual Dark Frames are
    > created is that since as you said, random noise would differ in each
    > DF but the effect of hot pixels wouldn't, by comparing the two,
    > practically the only thing the camera would subtract from the actual
    > image would be hot pixels, not the random contribution from noise.


    Again, this can be done in photoshop. Anyway, this is huge overkill for
    most photography. I only found such tricks to be of use in high ISO,
    long exposures (and stacking is better there).
     
    acl, Sep 27, 2006
    #13
  14. PeteD

    ASAAR Guest

    On 27 Sep 2006 04:16:21 -0700, acl wrote:

    >> Though unlikely to be implemented, it might be possible to design
    >> a camera such that most of the DFS benefit is obtained even when it
    >> is disabled. It would require enough memory to hold a small number
    >> of Dark Frame Profiles, maybe a one or two dozen, created by having
    >> the camera store what it has learned by creating Dark Frames over
    >> its range of shutter speeds and over a smaller range of
    >> temperatures. Every possible combination wouldn't be needed, as the
    >> camera could use interpolation to save time and space. But truly
    >> fussy photographers should have the ability to use an actual Dark
    >> Frame at any time, bypassing the DFPs if they wish. A default
    >> profile could be created for each camera by the manufacturer as part
    >> of the burn-in process, and this could also be available as a menu
    >> option to allow the DFPs to be refreshed as the camera ages.

    >
    > Yes, and the same could be achieved by doing it manually (taking a dark
    > frame and then subtracting it in eg photoshop). It's not as convenient,
    > though.


    Not only less convenient, but it might almost double the amount of
    time needed to take the shots that use DFS. That extra down time
    between shots can be quite annoying, even for those shots that don't
    exceed an exposure of a minute or two.


    >> One immediate benefit, even if all profiles were deleted, is that
    >> if you took a series of pictures in one setting where all of the
    >> exposures were the same, the result of the first DF would apply to
    >> each subsequent shot, saving the extra time as long as the exposure
    >> remains unchanged. There could be a flag displayed in the
    >> viewfinder when DFS is enabled, indicating if at the chosen
    >> exposure, the camera would actually have to take the time needed to
    >> create a Dark Frame. Another benefit if dual Dark Frames are
    >> created is that since as you said, random noise would differ in each
    >> DF but the effect of hot pixels wouldn't, by comparing the two,
    >> practically the only thing the camera would subtract from the actual
    >> image would be hot pixels, not the random contribution from noise.

    >
    > Again, this can be done in photoshop. Anyway, this is huge overkill for
    > most photography. I only found such tricks to be of use in high ISO,
    > long exposures (and stacking is better there).


    You mean by having the camera *save* extra frames for later
    processing? That would not only waste camera time, but memory card
    space as well. The point here wasn't to be able to produce the
    absolutely highest quality pictures possible, but to have a camera
    that could come very close by using a much more efficient method of
    Dark Frame Subtraction that would produce most of the benefits while
    at the same time eliminating nearly all of the photographer's wasted
    time doing nothing while the Dark Frames are being produced.
     
    ASAAR, Sep 27, 2006
    #14
  15. PeteD

    acl Guest

    ASAAR wrote:
    >
    > You mean by having the camera *save* extra frames for later
    > processing? That would not only waste camera time, but memory card
    > space as well. The point here wasn't to be able to produce the
    > absolutely highest quality pictures possible, but to have a camera
    > that could come very close by using a much more efficient method of
    > Dark Frame Subtraction that would produce most of the benefits while
    > at the same time eliminating nearly all of the photographer's wasted
    > time doing nothing while the Dark Frames are being produced.
    >


    Of course, the logical conclusion is to have a table of hot pixels and
    map them out!
     
    acl, Sep 27, 2006
    #15
  16. "acl" <> wrote:
    > ASAAR wrote:
    >>
    >> You mean by having the camera *save* extra frames for later
    >> processing? That would not only waste camera time, but memory card
    >> space as well. The point here wasn't to be able to produce the
    >> absolutely highest quality pictures possible, but to have a camera
    >> that could come very close by using a much more efficient method of
    >> Dark Frame Subtraction that would produce most of the benefits while
    >> at the same time eliminating nearly all of the photographer's wasted
    >> time doing nothing while the Dark Frames are being produced.
    >>

    >
    > Of course, the logical conclusion is to have a table of hot pixels and map
    > them out!


    That solves a different problem. DFS removes _repeatable_ noise patterns
    that occur at a particular temperature, ISO, and exposure time. Change any
    of those parameters, and the noise pattern changes. So you have to measure
    it at every shot where it might appear.

    That DFS aggravates the normal sensor noise is something I have neither
    heard before nor experienced myself, although (coincidentally) I haven't had
    need for DFS since moving to dSLRs from P&S cameras. My experience was that
    it worked very well in the P&S cameras I owned.

    David J. Littleboy
    Tokyo, Japan
     
    David J. Littleboy, Sep 27, 2006
    #16
  17. PeteD

    acl Guest

    David J. Littleboy wrote:
    > "acl" <> wrote:
    >>ASAAR wrote:
    >>> You mean by having the camera *save* extra frames for later
    >>>processing? That would not only waste camera time, but memory card
    >>>space as well. The point here wasn't to be able to produce the
    >>>absolutely highest quality pictures possible, but to have a camera
    >>>that could come very close by using a much more efficient method of
    >>>Dark Frame Subtraction that would produce most of the benefits while
    >>>at the same time eliminating nearly all of the photographer's wasted
    >>>time doing nothing while the Dark Frames are being produced.
    >>>

    >>Of course, the logical conclusion is to have a table of hot pixels and map
    >>them out!

    >
    > That solves a different problem. DFS removes _repeatable_ noise patterns
    > that occur at a particular temperature, ISO, and exposure time. Change any
    > of those parameters, and the noise pattern changes. So you have to measure
    > it at every shot where it might appear.


    My hot pixels are repeatable. Of course, to remove the effects of heat
    from amplifiers etc, you do need DFS. But anyway, it was a joke: trying
    to minimise amount of info stored divided by convenience leads to a
    table of coordinates stored.

    >
    > That DFS aggravates the normal sensor noise is something I have neither
    > heard before nor experienced myself, although (coincidentally) I haven't had
    > need for DFS since moving to dSLRs from P&S cameras. My experience was that
    > it worked very well in the P&S cameras I owned.



    Well, I saw it in my SLR but not in the compact (which has so much noise
    under these conditions that a mere doubling is not really perceptible).
    If you think about, it would be extremely strange if this did not
    happen. Try adding up N random variables between -1 and 1 (say); if you
    repeat the experiment many times, you're as likely to get something
    positive as negative, and on average of course it should be zero; but on
    a given trial, how often do you actually get zero? (Note that we didn't
    divide by N, which would correspond to averaging the dark frames rather
    than just adding them).
     
    acl, Sep 27, 2006
    #17
  18. "acl" <> wrote:
    >>
    >> That DFS aggravates the normal sensor noise is something I have neither
    >> heard before nor experienced myself, although (coincidentally) I haven't
    >> had need for DFS since moving to dSLRs from P&S cameras. My experience
    >> was that it worked very well in the P&S cameras I owned.

    >
    > Well, I saw it in my SLR but not in the compact (which has so much noise
    > under these conditions that a mere doubling is not really perceptible). If
    > you think about, it would be extremely strange if this did not happen. Try
    > adding up N random variables between -1 and 1 (say); if you repeat the
    > experiment many times, you're as likely to get something positive as
    > negative, and on average of course it should be zero; but on a given
    > trial, how often do you actually get zero? (Note that we didn't divide by
    > N, which would correspond to averaging the dark frames rather than just
    > adding them).


    When the inbox gets less crazed I'll look into this. With the Sony F707,
    some number of very bright pixels would show up without DFS, and with DFS
    they'd be gone with no apparent degradation, in fact with very nice images
    (macros at ISO 100 and long exposures). So the DFS signal wasn't really a
    "noise" signal on every pixel, it was some number of hot pixels getting
    subtracted out. That is, I think you're right were it a typical random noise
    signal, but on the Sony it had different characteristics. Anyway, thanks for
    pointing this out; something else interesting to figure out.

    Canon uses a different logic on the DFS control than the more
    straightforward Sony system, so I haven't checked it out on the Canons yet.
    (I think it's either always on or on only when the camera thinks it needs
    it.) The Sony had a cute glitch: it could only do DFS if you told it what
    the exposure was, so DFS didn't happen in Av and full auto mode. The first
    time I did a long exposure in Av mode, I got a nasty shock.

    David J. Littleboy
    Tokyo, Japan
     
    David J. Littleboy, Sep 27, 2006
    #18
  19. PeteD

    ASAAR Guest

    On Wed, 27 Sep 2006 16:25:05 +0200, acl wrote:

    >> You mean by having the camera *save* extra frames for later
    >> processing? That would not only waste camera time, but memory card
    >> space as well. The point here wasn't to be able to produce the
    >> absolutely highest quality pictures possible, but to have a camera
    >> that could come very close by using a much more efficient method of
    >> Dark Frame Subtraction that would produce most of the benefits while
    >> at the same time eliminating nearly all of the photographer's wasted
    >> time doing nothing while the Dark Frames are being produced.

    >
    > Of course, the logical conclusion is to have a table of hot pixels and
    > map them out!


    That would help only for stuck pixels, not for those that are time
    or temperature sensitive, or (guessing now) those that behave in a
    very nonlinear fashion. And except for a few cameras that allow
    remapping in the field, most cameras, I think, must be sent back to
    the manufacturer to have newly stuck/hot pixels mapped out.
     
    ASAAR, Sep 27, 2006
    #19
  20. Paul Rubin <http://> writes:

    >But in this case the camera could compensate by averaging together a
    >bunch of dark frames. The random components would cancel out and the
    >systematic components would stay.


    That's what astronomers do. But the bunch of dark frames need to be
    captured with the same sensor temperature and exposure time as will be
    used for the image-forming exposure. Astronomers use cooled CCDs whose
    temperature remains constant, eliminating the one variable, and they
    also tend to shoot many images with the same exposure time. So they can
    shoot a bunch of dark frames at the beginning of an observing session
    and use them for all subsequent images.

    For a camera with an uncooled CCD and varying exposure times, the
    camera would have to shoot several dark frames after *each* exposure,
    greatly adding to exposure time. So they shoot zero or one dark frame.

    Dave
     
    Dave Martindale, Sep 28, 2006
    #20
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