What are these blue and white dots

Discussion in 'Digital Photography' started by ronviers, Mar 11, 2007.

  1. ronviers

    ronviers Guest


    A thumbnail of the file can be seen here:


    Where you can look at a small abstract of the EXIF data to the right
    by using the 'more info' link. There are arrows pointing to instances
    of the problem but you will need to use the 'Download Photo' link to
    get a look at the dots. The link is to a 1.8Mb file so don't click on
    it unless you really want to see.
    This image has not been sharpened.

    ronviers, Mar 11, 2007
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  2. I was going to guess "noise", but they don't look like digital photo
    noise to me in the big photo. There's also a lot of dust and stuff on
    the actual microscope, it looks like, but I see a relatively few, fairly
    big, blue and white spots in addition to the dust, and they are where
    the arrows point, so I think I'm looking at the same problem you're
    looking at.

    Possibly hot pixels in your sensor? I haven't experienced that problem
    so I'm not sure I know what they look like.

    Any chance of editing mis-clicks or something? Looked like nothing
    changed at the view level, but really a pixel or two got munged?

    Or something I have no clue about.

    I'll follow this thread with interest!
    David Dyer-Bennet, Mar 11, 2007
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  3. Hmm. I've never seen anything quite like that... But what's with the
    weird exposure time - 717, or 17 seconds? Huh??

    I'm also a little puzzled by the lack of any details to help
    troubleshoot - do other images show this, or only this one? Have you
    shot other stuff at long exposures? Do these spots always appear at
    the same locations? Surely there are other clues..

    By your description I was just expecting to see a few hot-pixels, but
    they are just everywhere... And is this a 'straight' photo? or is
    something else funny going on? The reason I ask is that you say there
    is no sharpening, and yet the white spots often have severe ringing
    artefacts as one might expect from oversharpening. The image itself
    looks a little soft, yet many of the artefacts are very sharp indeed.

    In the absence of a lot more info, I'm guessing some sort of
    processing/firmware problem. Could there have been a lot of
    electrical interference where this was taken?
    mark.thomas.7, Mar 11, 2007
  4. ronviers

    ASAAR Guest

    At first I thought it might be small clumps of hot pixels. But
    there are *many* smaller ones, and the color fringing along many of
    the edge transitions between the microscope and the bright
    background are also a similar blue. Did you notice that the blue
    and white dots can be seen practically everywhere, including most of
    the white background? My guess is that it's related to having such
    a long exposure (717 seconds) and possibly the small f/22 aperture.
    If you didn't already do it, could you take another shot using dark
    frame subtraction to see if it makes any difference? I've read that
    DSLRs have much better sensitivity than film, but sensors don't
    handle extremely long exposures as well as film.
    ASAAR, Mar 11, 2007
  5. ronviers

    Mark² Guest

    Might be hot pixels, which often show up as bright, colored blobs of blue,
    green, etc.
    Mark², Mar 11, 2007
  6. ronviers

    ronviers Guest

    Here is a dark version of the setup.


    If you pin register the two you will notice that all the dots are
    present and at the same locations in both images.

    BTW, the images are unedited and the RAW settings were copied from the
    ECE raw and pasted to the dark raw so it should be valid - of course
    they are jpgs but the tiffs are too big to upload.

    ronviers, Mar 11, 2007
  7. ronviers

    gpaleo Guest

    Thermal noise from a long exposure.
    Using some HDR software, you can average or sum multiple, same, less long
    exposures and effectively get rid of the noise.
    gpaleo, Mar 11, 2007
  8. ronviers

    ASAAR Guest

    My humble photo software doesn't do dark subtraction. I meant
    that it might be useful to have the camera do the dark frame
    subtraction. But as the dots are in the same positions, and the
    exposure was the same, it seems that you've just gone beyond the
    Rebel's ability to handle long exposures, at least at ambient
    temperature used for the shots. Care to try again in a walk-in
    freezer? <g> I'd guess that by getting into very long exposure
    times your sensor's individual sensels' sensitivities aren't equally
    linear, maybe due to nonuniform leakage. If anyone has better
    suggestions for you and your camera it might be one of the several
    rpd'ers more knowledgeable in long exposure astrophotography, R.
    Clark's name coming to mind, among others.

    And with my dialup connection, waaaay too big to download. :)
    BTW, the dark frame is still downloading, but it has already
    exceeded 1.5MB, close to the size of the original photo. Even with
    the lowest jpeg compression ratio I'd have thought that a dark frame
    would be highly compressible? It just finished, but I'm still
    surprised that the 1,661kb dark image wasn't much smaller than the
    1,909kb original. It guess that there's lots of very faint shadow
    ASAAR, Mar 11, 2007
  9. ronviers

    ronviers Guest

    Thanks to everyone for their help. Capturing a dark image with that
    setup was the definitive test. I was just pushing the sensor too far.
    I reworked the lighting and got an acceptable shot staying within the
    lens's comfort f range but loosing some interest in perspective. The
    rules for this image require no out of camera processing so HDR and
    noise reduction were not an option.
    Those low light shots are tricky.

    ronviers, Mar 11, 2007
  10. I'm still not sure you understood ASAAR's suggestion of using in camera
    dark frame subtraction. Your reply was unclear if you actually used this.
    Surely your Canon has this feature.
    Ed Ruf (REPLY to E-MAIL IN SIG!), Mar 11, 2007
  11. ronviers

    ronviers Guest

    Not this model, that feature was added later. Even doing simple
    subtraction in Photoshop made very little improvement (not that that
    it is allowed) in the shadows and made the high key areas worse, so an
    additional mask would be needed, because so little information
    remained in the dots. I would need some third party NR software to try
    to salvage the original setup.

    ronviers, Mar 11, 2007
  12. That's sad. Even my old P&S CP-5700 had this feature, let alone my old D70.
    I don't believe 3rd party noise reduction programs will deal with hot
    Ed Ruf (REPLY to E-MAIL IN SIG!), Mar 11, 2007
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