Nikon D70 long exposure - purple blobs?

Discussion in 'Digital Photography' started by Kevin, Aug 2, 2005.

  1. Kevin

    Kevin Guest

    Hey guys! So I just recently got a D70 and took it to a summer camp for a
    few weeks. Got some great photos with it (in my opinion anyway, I'm still
    very much a rank amateur...)

    My brother and I tried taking some star photos, such as this one:

    http://www.cherith.ca/gallery/week1/aai

    (ISO 1600, about a 2-minute exposure in bulb mode. The linked photo is
    very much shrunken down from the original camera JPEG, but the problem is
    still very visible)

    We notice that on many of these long exposure shots, there are purple
    blobs in the upper left corner. Seems like the longer the exposure, the more
    pronounced the blobs are.

    What is this? (Camera itself, sensor, lens?) Is this normal, or do I have a
    bad unit? I'd hate to think I have a defective unit, yet I'd also hate to
    think this is normal for the D70, as we were hoping to experiment with more
    shots like this in the future...

    I also notice a strong purple cast in some of my photos, though I have not
    experimented to see what circumstances they show up in.

    Any ideas? Thanks for your time...
    Kevin, Aug 2, 2005
    #1
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  2. Kevin

    Sheldon Guest

    "Kevin" <> wrote in message
    news:dcmilb$h24$...
    > Hey guys! So I just recently got a D70 and took it to a summer camp for a
    > few weeks. Got some great photos with it (in my opinion anyway, I'm still
    > very much a rank amateur...)
    >
    > My brother and I tried taking some star photos, such as this one:
    >
    > http://www.cherith.ca/gallery/week1/aai
    >
    > (ISO 1600, about a 2-minute exposure in bulb mode. The linked photo is
    > very much shrunken down from the original camera JPEG, but the problem is
    > still very visible)
    >
    > We notice that on many of these long exposure shots, there are purple
    > blobs in the upper left corner. Seems like the longer the exposure, the
    > more
    > pronounced the blobs are.
    >
    > What is this? (Camera itself, sensor, lens?) Is this normal, or do I have
    > a
    > bad unit? I'd hate to think I have a defective unit, yet I'd also hate to
    > think this is normal for the D70, as we were hoping to experiment with
    > more
    > shots like this in the future...
    >
    > I also notice a strong purple cast in some of my photos, though I have not
    > experimented to see what circumstances they show up in.
    >
    > Any ideas? Thanks for your time...


    It looks like some stray light or a reflection of somekind. Do you get the
    problem regardless of where you aim the camera? To see if it's the camera
    just repeat the exposure with the lens cap on. Keep us posted.

    Sheldon
    Sheldon, Aug 2, 2005
    #2
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  3. Kevin

    Guest

    Kevin wrote:

    > Hey guys! So I just recently got a D70 and took it to a summer camp for a
    > few weeks. Got some great photos with it (in my opinion anyway, I'm still
    > very much a rank amateur...)
    >
    > My brother and I tried taking some star photos, such as this one:
    >
    > http://www.cherith.ca/gallery/week1/aai
    >
    > (ISO 1600, about a 2-minute exposure in bulb mode. The linked photo is
    > very much shrunken down from the original camera JPEG, but the problem is
    > still very visible)
    >
    > We notice that on many of these long exposure shots, there are purple
    > blobs in the upper left corner. Seems like the longer the exposure, the more
    > pronounced the blobs are.
    >
    > What is this? (Camera itself, sensor, lens?) Is this normal, or do I have a
    > bad unit? I'd hate to think I have a defective unit, yet I'd also hate to
    > think this is normal for the D70, as we were hoping to experiment with more
    > shots like this in the future...
    >
    > I also notice a strong purple cast in some of my photos, though I have not
    > experimented to see what circumstances they show up in.
    >
    > Any ideas? Thanks for your time...


    Don't know if the other pictures in the series you took were with your
    D70, but I didn't notice any purple hue in them...at least not at that
    resolution. Anyway, I took some night shots w/ my D70s not long ago
    that turned out pretty noisy, and I figured out it was due to two
    things: 1) I didn't have long exposure noise reduction (Menu->Camera
    Icon->Long Exposure NR) on, and 2) I had VR on in my lens (24-120mm VR
    AF-S Nikkor) w/ a tripod...a no-no. Next time out, no VR w/ long
    exposure NR, and the shots were fine. Nothing as long as 2:00, but
    fine, nonetheless.

    Note: Once you are done shooting longer exposures, make sure and turn
    the NR back off - it affects write time in normal exposures, IIRC.
    Also, be patient while it writes to the card in NR mode, as well - it
    takes a few seconds to do so, so don't worry about the longer wait to
    review the shot(s). HTH...

    Good luck,

    Ben
    , Aug 2, 2005
    #3
  4. Kevin

    Ryan Guest

    > We notice that on many of these long exposure shots, there are purple
    > blobs in the upper left corner. Seems like the longer the exposure, the more
    > pronounced the blobs are.



    It looks like light pollution to me. With a long exposure, the
    slightest light source makes a big difference, at least in my
    experience.

    I shoot lightning a bit and a street light down the block gives me loads
    of trouble.


    Did you use a lens hood? If not, do so. If there are light sources
    within direct ray view, try to find a shadow and put the front of your
    lens in that shadow. This can be a tree, a utility pole, an upright on
    a porch, a mailbox, whatever you can find. If that is stray light
    from a nearby city, then... I don't know, hide behind a wall perhaps?

    I forget the number, but I think around 40 seconds or so, is the point
    at which star motion blur begins to show on a non moving exposure.
    Perhaps someone can correct that if I am mistaken.
    Ryan, Aug 2, 2005
    #4
  5. Kevin

    Kevin Guest

    Sheldon <> wrote:
    > It looks like some stray light or a reflection of somekind. Do you get the
    > problem regardless of where you aim the camera? To see if it's the camera
    > just repeat the exposure with the lens cap on. Keep us posted.


    Hi Sheldon,

    Thanks. I'll try the lenscap-on trick later to see what happens. Meantime
    I've uploaded a couple of other photos to http://www.cherith.ca/jay, where
    you can see the purple-blob problem is occurring regardless of camera
    orientation.

    I'll also try the NR feature and see what happens..
    Kevin, Aug 2, 2005
    #5
  6. Kevin

    Kevin Guest

    Ryan <> wrote:
    > I forget the number, but I think around 40 seconds or so, is the point
    > at which star motion blur begins to show on a non moving exposure.
    > Perhaps someone can correct that if I am mistaken.


    Around that. I was mistaken; though we did take exposures as long as 2
    minutes (which definitely show signs of star motion), the picture I posted
    was probably only 30-45 seconds.

    Anyway, based on advice from Sheldon, I did some more tests, and it looks
    like it is not light pollution. I did 30-second and 1-minute exposures with
    the lens cap on, just now in my home, and the same purple blobs appear in the
    same positions. When I turn on noise reduction, the purple blobs are no
    longer present in the image. At least I have a workaround, but it would
    still be nice to know whether this is considered "normal" behaviour for a
    D70, or whether I should take it back...

    Thanks for your help and advice.
    Kevin, Aug 2, 2005
    #6
  7. Kevin

    Pete D Guest

    "Kevin" <> wrote in message
    news:dcmqnc$q15$...
    > Ryan <> wrote:
    >> I forget the number, but I think around 40 seconds or so, is the point
    >> at which star motion blur begins to show on a non moving exposure.
    >> Perhaps someone can correct that if I am mistaken.

    >
    > Around that. I was mistaken; though we did take exposures as long as 2
    > minutes (which definitely show signs of star motion), the picture I posted
    > was probably only 30-45 seconds.
    >
    > Anyway, based on advice from Sheldon, I did some more tests, and it looks
    > like it is not light pollution. I did 30-second and 1-minute exposures
    > with
    > the lens cap on, just now in my home, and the same purple blobs appear in
    > the
    > same positions. When I turn on noise reduction, the purple blobs are no
    > longer present in the image. At least I have a workaround, but it would
    > still be nice to know whether this is considered "normal" behaviour for a
    > D70, or whether I should take it back...
    >
    > Thanks for your help and advice.


    It is quite normal, many cameras do the same thing and the software in their
    camera will fix things up, use a lower ISO and noise reduction for long
    exposures, you don't need the higher ISO anyway, thats why you are using a
    long exposure.
    Pete D, Aug 2, 2005
    #7
  8. Kevin

    Ed Ruf Guest

    On 2 Aug 2005 01:35:39 GMT, in rec.photo.digital Kevin
    <> wrote:

    >Hey guys! So I just recently got a D70 and took it to a summer camp for a
    >few weeks. Got some great photos with it (in my opinion anyway, I'm still
    >very much a rank amateur...)
    >
    >My brother and I tried taking some star photos, such as this one:
    >
    >http://www.cherith.ca/gallery/week1/aai
    >
    >(ISO 1600, about a 2-minute exposure in bulb mode. The linked photo is
    >very much shrunken down from the original camera JPEG, but the problem is
    >still very visible)
    >
    >We notice that on many of these long exposure shots, there are purple
    >blobs in the upper left corner. Seems like the longer the exposure, the more
    >pronounced the blobs are.
    >
    >What is this? (Camera itself, sensor, lens?) Is this normal, or do I have a
    >bad unit? I'd hate to think I have a defective unit, yet I'd also hate to
    >think this is normal for the D70, as we were hoping to experiment with more
    >shots like this in the future...
    >
    >I also notice a strong purple cast in some of my photos, though I have not
    >experimented to see what circumstances they show up in.
    >
    >Any ideas? Thanks for your time...


    Try the les cap photo as Sheldon suggested. Also, don't forget to use the
    viewfinder cover as any stray light captured by this will be recorded as
    well. The purple cast you mention is in normal exposure shots as well?

    Other things to possibly try, upgrade to latest firmware, use the factory
    reset switch on the bottom.
    ----------
    Ed Ruf Lifetime AMA# 344007 ()
    See images taken with my CP-990/5700 & D70 at
    http://edwardgruf.com/Digital_Photography/General/index.html
    Ed Ruf, Aug 2, 2005
    #8
  9. Kevin

    Martin Brown Guest

    Sheldon wrote:

    > "Kevin" <> wrote in message
    > news:dcmilb$h24$...
    >
    >>Hey guys! So I just recently got a D70 and took it to a summer camp for a
    >>few weeks. Got some great photos with it (in my opinion anyway, I'm still
    >>very much a rank amateur...)
    >>
    >>My brother and I tried taking some star photos, such as this one:
    >>
    >>http://www.cherith.ca/gallery/week1/aai
    >>
    >>(ISO 1600, about a 2-minute exposure in bulb mode. The linked photo is
    >>very much shrunken down from the original camera JPEG, but the problem is
    >>still very visible)
    >>
    >>We notice that on many of these long exposure shots, there are purple
    >>blobs in the upper left corner. Seems like the longer the exposure, the
    >>more
    >>pronounced the blobs are.
    >>
    >>What is this? (Camera itself, sensor, lens?) Is this normal, or do I have
    >>a
    >>bad unit? I'd hate to think I have a defective unit, yet I'd also hate to
    >>think this is normal for the D70, as we were hoping to experiment with
    >>more
    >>shots like this in the future...
    >>
    >>I also notice a strong purple cast in some of my photos, though I have not
    >>experimented to see what circumstances they show up in.
    >>
    >>Any ideas? Thanks for your time...

    >
    >
    > It looks like some stray light or a reflection of somekind. Do you get the
    > problem regardless of where you aim the camera? To see if it's the camera
    > just repeat the exposure with the lens cap on. Keep us posted.


    It is and it is entirely normal behaviour in an uncooled CCD camera on
    long exposures. IR photons and thermal effects from the on chip read out
    control electronics affect one corner of the display. Modern chips are
    better than they used to be. Take a dark frame shot for the same time
    exposure of the inside of your lens cap and subtract it to get something
    closer to the true image. NB ideally taken at the same time with the
    sensor at roughly the same temperature.

    The Kodak DC-120 was one of the first to allow long exposures and
    suffered from considerably more warm corner effects on 16s exposures.

    http://www.nezumi.demon.co.uk/astro/dc120/dc120.htm

    To get away from the problem you need to actively cool the chip -
    impractical on consumer cameras but routine in scientific work.

    Regards,
    Martin Brown
    Martin Brown, Aug 2, 2005
    #9
  10. Kevin

    Owamanga Guest

    On Tue, 02 Aug 2005 05:38:09 -0400, Ed Ruf <>
    wrote:

    >Try the les cap photo as Sheldon suggested. Also, don't forget to use the
    >viewfinder cover as any stray light captured by this will be recorded as
    >well. The purple cast you mention is in normal exposure shots as well?


    I was under the impression that the viewfinder cover's purpose is to
    allow for accurate metering without viewfinder light effecting the
    outcome. Once the shutter is tripped the mirror blocks the light from
    the viewfinder doesn't it?

    Anyway, as others have suggested, this is caused by on-camera chip's
    thermal radiation affecting the sensor. Dark frame subtraction can
    help, but it, added to the hot-pixel filter stuff the D70 does
    automatically doesn't lend itself to scientific-quality astronomy
    images.

    I remember reading somewhere that there is a trick of turning the
    camera off during it's RAW write that will prevent it from running the
    hot pixel filter (which kills most of the fainter stars). I don't
    recall the details though.

    --
    Owamanga!
    http://www.pbase.com/owamanga
    Owamanga, Aug 2, 2005
    #10
  11. Kevin

    Mike Guest

    "Owamanga" <> a écrit dans le message de
    news: ...
    > On Tue, 02 Aug 2005 05:38:09 -0400, Ed Ruf <>
    > wrote:


    >
    > I was under the impression that the viewfinder cover's purpose is to
    > allow for accurate metering without viewfinder light effecting the
    > outcome. Once the shutter is tripped the mirror blocks the light from
    > the viewfinder doesn't it?


    It should, but how to be absolutely sure? Some mirrors are smaller than they
    used to be... (more recessed in more compact models?)
    According to Canon (EOS D. Rebel for example) the use of the viewfinder
    cover should be seriously considered.

    Anyway, as said here after, if the flares show up in every night long
    exposure regardless of the orientation of close sources of artificial light,
    the viewfinder cover should not be the solution.


    >
    > Anyway, as others have suggested, this is caused by on-camera chip's
    > thermal radiation affecting the sensor. Dark frame subtraction can
    > help, but it, added to the hot-pixel filter stuff the D70 does
    > automatically doesn't lend itself to scientific-quality astronomy
    > images.
    >
    > I remember reading somewhere that there is a trick of turning the
    > camera off during it's RAW write that will prevent it from running the
    > hot pixel filter (which kills most of the fainter stars). I don't
    > recall the details though.
    >
    > --
    > Owamanga!
    > http://www.pbase.com/owamanga
    Mike, Aug 2, 2005
    #11
  12. Kevin

    Ed Ruf Guest

    On Tue, 02 Aug 2005 12:38:55 GMT, in rec.photo.digital Owamanga
    <> wrote:

    >On Tue, 02 Aug 2005 05:38:09 -0400, Ed Ruf <>
    >wrote:
    >
    >>Try the les cap photo as Sheldon suggested. Also, don't forget to use the
    >>viewfinder cover as any stray light captured by this will be recorded as
    >>well. The purple cast you mention is in normal exposure shots as well?

    >
    >I was under the impression that the viewfinder cover's purpose is to
    >allow for accurate metering without viewfinder light effecting the
    >outcome. Once the shutter is tripped the mirror blocks the light from
    >the viewfinder doesn't it?


    I stand corrected. Should have checked the manual before putting foot in
    mouth.
    ----------
    Ed Ruf Lifetime AMA# 344007 ()
    See images taken with my CP-990/5700 & D70 at
    http://edwardgruf.com/Digital_Photography/General/index.html
    Ed Ruf, Aug 2, 2005
    #12
  13. Kevin

    Sheldon Guest

    Okay, this thread was driving me nuts, so I took a few 2 minute exposures
    with my D70 and the lens cover on. Completely black image from side to side
    and corner to corner. Some were taken with the viewfinder convered, some
    not.

    Sheldon
    Sheldon, Aug 3, 2005
    #13
  14. Kevin

    Kevin Guest

    Sheldon <> wrote:
    > Okay, this thread was driving me nuts, so I took a few 2 minute exposures
    > with my D70 and the lens cover on. Completely black image from side to side
    > and corner to corner. Some were taken with the viewfinder convered, some
    > not.


    Sheldon,

    Glad to hear that.

    If there are any other D70 users out there who would be willing, I would
    appreciate additional data points. Please let me know if you have any
    noise problems with long exposures.

    Thanks guys, you've been most helpful.
    Kevin, Aug 3, 2005
    #14
  15. Kevin wrote:
    > Sheldon <> wrote:
    >
    >>Okay, this thread was driving me nuts, so I took a few 2 minute exposures
    >>with my D70 and the lens cover on. Completely black image from side to side
    >>and corner to corner. Some were taken with the viewfinder convered, some
    >>not.

    >
    >
    > Sheldon,
    >
    > Glad to hear that.
    >
    > If there are any other D70 users out there who would be willing, I would
    > appreciate additional data points. Please let me know if you have any
    > noise problems with long exposures.
    >
    > Thanks guys, you've been most helpful.
    >
    >

    Ok, I had a bash as well. At 2 minutes with NR Off & lens cap on I got a
    purple flare at the top left corner while viewing it on the camera's LCD.

    With NR On there was no purple flare.

    BUT .... looking at both pictures ( RAW ) in Nikon Capture there was no
    purple flare in either of the shots ... now I'm lost ??
    Bernard
    Bernard Rother, Aug 3, 2005
    #15
  16. Kevin

    Eager Guest

    Sheldon --

    Important detail: was NR on or off??

    -=-Joe


    In article <>, Sheldon
    <> wrote:

    > Okay, this thread was driving me nuts, so I took a few 2 minute exposures
    > with my D70 and the lens cover on. Completely black image from side to side
    > and corner to corner. Some were taken with the viewfinder convered, some
    > not.
    Eager, Aug 3, 2005
    #16
  17. Kevin <> wrote:
    > We notice that on many of these long exposure shots, there are purple
    > blobs in the upper left corner. Seems like the longer the exposure, the more
    > pronounced the blobs are.


    As others have pointed out, the simplest solution is to turn
    on noise reduction.
    See also
    http://www.ooblick.com/text/noise-reduction/
    for some pictures of this phenomenon.

    --
    Andrew Arensburger, Systems guy University of Maryland
    Office of Information Technology
    A problem can be found for almost every solution.
    Andrew Arensburger, Aug 16, 2005
    #17
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