15-sec exposures of night sky yields too many stars?

Discussion in 'Digital Photography' started by Juan R. Pollo, Dec 3, 2003.

  1. Hello, all

    I mounted my Canon A40 on a tripod and aimed it at the South Florida night
    sky (west Kendall, edge of Everglades). I took many 15 sec exposures at
    different angles and orientations (ISO 50, 2 sec self-timer to minimize
    shake). I was amazed at the results until I realized that fences and walls
    also had "stars" on them. Any ideas as to what happened? Did the camera pick
    up actual light, or is this what they call "noise"? Thanks for your help.

    Juan R. Pollo, Dec 3, 2003
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  2. Juan R. Pollo

    Don Coon Guest

    Definitely noise. Try shorter exposures and you'll see fewer "stars." I'm
    not sure if 15 seconds is long enough to see star movement but if it is you
    will be able to see which are stars and which are noise.
    Don Coon, Dec 3, 2003
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  3. Juan R. Pollo

    Ben Thomas Guest

    I've noticed similar "dots" when doing long exposures at night with my Kodak
    DX6490. The dark side of the moon had a star in front of it!

    Ben Thomas - Software Engineer - UNICO Computer Systems
    Melbourne, Australia

    Opinions, conclusions, and other information in this message that do not
    relate to the official business of my employer, UNICO Computer Systems,
    shall be understood as neither given nor endorsed by it.
    Ben Thomas, Dec 3, 2003
  4. That's "noise" and it's why astronomers use special cooled CCDs. A CCD has
    to be cooled below freezing to get the noise down to levels acceptable for
    astronomy. However, you may be able to get around the problem, partly, by
    subtracting a "dark frame" (an identical exposure with the lenscap on) from
    the image.
    Michael A. Covington, Dec 3, 2003
  5. Juan R. Pollo

    Ron Hunter Guest

    Yep, noise.
    Ron Hunter, Dec 3, 2003
  6. Juan R. Pollo

    MikeWhy Guest

    Possibly noise, but seems less likely at ISO 50. If they're all in the same
    place from frame to frame, they might be stuck pixels.
    MikeWhy, Dec 3, 2003
  7. Juan R. Pollo

    DHB Guest

    check your thread about "landscape mode" where I responded to that

    As for your question about possible noise with your Canon A40 set to ISO
    50 & 15 sec. exposures at the night sky, without seeing your pictures I
    can't say what it might be but is does not sound like noise to me.

    The A40 automatically applies "dark frame subtraction" noise reduction
    to any shutter length of 1.3 sec. or longer. I have taken hundreds of 5-15
    sec. long exposures @ ISO 50 both indoors & outdoors & have yet to see
    anything close to objectionable or even noticeable noise.

    If you want to see what CCD noise on your A40 looks like, set the ISO to
    400 & take a 1 sec. exposure of the night sky or anything that won't result
    in an over exposure. The noise will be there because of the high ISO
    setting & the fact that the noise reduction will not be applied at this
    shutter speed. It's possible that you may not have manually set your flash
    to "off", if the flash fired during this long exposure it very likely
    illuminated dust, lint or even pollen that may have been floating by.

    I know people are probably thinking the night air is clean but you need
    only record video with an near infrared sensitive camera & a near infrared
    LED cluster illuminator & you will be amazed at how much "tiny little stuff"
    floats by & is illuminated so clearly. If you see very little, stand in
    from of the camera & tap on your shirt or jacket & lots of bright tiny
    specks of dust & lint will be released into the air & float by the camera.
    It sure amazed me the first time I noticed it, I though it looked like
    random snow flakes floating by but I knew this could not be since it was an
    80 degree august night.

    It's a strange world out there & our human eyes only see a tiny spectrum
    of what's out there. Insects & nocturnal animals see higher & lower
    frequencies than we do so things look different to them & that's also why
    they can see at night without a problem. Digital CCD & CMOS sensors can
    also see more than we can with our eyes especially in the near infrared so
    that's why they all have infrared block filters built into them or it would
    mess-up the color rendition from what we see & consider "normal".

    Best of luck with your A40 it's a great little camera. If you really
    want to see what it's capable of in existing light situations, take it with
    you to a restaurant. Set the "ISO to 50", white balance to "tungsten",
    photo effect to "vivid", "flash off" & use the 2 sec. self timer. Sit it
    near the end of the table or across the top of a glass, aim it in the
    general direction you want with it on "full wide angle" & press the shutter.
    Start with a 5 sec. exposure & work up from there or down if needed.
    Anything that moves will be strongly blurred but the ambiance of the place
    will be clearly captured. To be less distracting to others I turn off the
    camera sounds & even place a peace of black electrical tape over the flash
    so that the self timer flashing is not seen.

    Enjoy, it's a great little camera as is the A60 which replaced it. My
    A40 has over 3,600 pictures on it & it's going to continue to be used until
    1 of us stops working!

    Respectfully, DHB
    DHB, Dec 3, 2003
  8. Thank you for your thoughtful reply, I posted one of the shots in question
    at http://www.photosig.com/go/photos/view?id=1091875

    Unfortunately I can only post once every 72 hours, I would have liked to
    post a couple more.

    I tend to go with the noise theory because any dust specks would have to
    stand still for a couple of seconds at least to create an impression. The
    other possibility is stray headlight reflections from the walls, but that
    doesn't explain the specks on the inside side of a 6 foot fence.

    Juan R. Pollo, Dec 3, 2003
  9. Juan R. Pollo

    Jim Townsend Guest

    It is noise..

    This is normal with digital cameras. It happens with digital SLR's as well..
    except it takes about 90 seconds before the dots start showing up.
    Jim Townsend, Dec 3, 2003
  10. Juan R. Pollo

    DHB Guest

    Now that I have seen 1 of your pictures, I must admit that it does look
    like random CCD generated noise. Am a bit surprised because mine produces
    such clean results but then again I am shooting in low light situations &
    not "almost no light" situations as your night sky shot basically was.

    Now you have my curiosity up & I am going to have to try to reproduce
    your picture with my A40 just to see if mine exhibits the same or similar
    noise artifacts. Like it or not I am just 1 of those people that don't like
    unsolved mysteries, so now I will have to try to recreate similar conditions
    & try a few shots with my A40 in order to satisfy my curiosity! Yes I'll
    let you know what mine did & may do likewise with my A70 just for any added
    insight that it might yield.

    The floating dust or etc. was only a possibility if you had forgotten to
    turn off the flash because if it fired it would have illuminated anything
    floating within about 15 ft or less of the camera even at ISO 50. But if
    the flash did not fire than that suggested possibility has been ruled out.

    Do hope you try what I suggested in a restaurant or even a bar/lounge
    because the effects are often quite interesting & surprisingly clear with
    deeply rich colors.

    Those lead weights are tugging at my eye lids so I had best let them
    have there way & get some sleep.

    Respectfully, DHB
    DHB, Dec 3, 2003
  11. Not so--you can earn more posting slots by making critiques that other
    people consider helpful.

    Louise Bremner (log at gol dot com)
    If you want a reply by e-mail, don't write to my Yahoo address!
    Louise Bremner, Dec 3, 2003
  12. Digital noise is *very* temperature-dependent and you will notice a
    difference even between 70 F and 40 F.

    Clear skies,

    Michael Covington -- www.covingtoninnovations.com
    Author, Astrophotography for the Amateur
    and (new) How to Use a Computerized Telescope
    Michael A. Covington, Dec 3, 2003
  13. Juan R. Pollo

    Don Coon Guest

    On my DSLR (10D) with the lens cap on, I got 12 "dots" in 164 seconds at ISO
    1600. 12 out 6,291,456!

    I did this to do a blackframe subtraction since I had taken a few shots of
    the Northern Lights at 1600 before I realized it and set it back to 100.
    Don Coon, Dec 3, 2003
  14. Juan R. Pollo

    zbzbzb Guest

    I mounted my Canon A40 on a tripod and aimed it at the South Florida
    On my Canon A80 at its max of 15 seconds at ISO 400 I get no "dots."
    zbzbzb, Dec 3, 2003
  15. Juan R. Pollo

    Jim Townsend Guest

    You'll find that you get more white pixels with the lens cap off. These aren't
    stuck pixels, but more accurately, they're saturated pixels.

    The 'saturated' pixels in long exposures are sensor locations that can't bleed
    off the charge created by light falling on them.

    With no light, (lens cap on), the charge doesn't occur. With the cap off, even
    in low light, there's enough to saturate some of the sensors and cause white

    Try a 5 minute shot with lens cap on, then another 5 minute shot under low
    light using ISO100 at around f/22. You'll see many more 'hot' pixels on that
    Jim Townsend, Dec 3, 2003
  16. Maybe we should reverse the order to eliminate the CCD temperature factor. A
    cooler CCD will have less noise.

    Juan R. Pollo, Dec 3, 2003
  17. Juan R. Pollo

    Barry Smith Guest

    In message <[email protected]_s03>
    A quick guide to 'star trails', or rather avoiding them, assuming the
    camera is fixed -

    With a 135mm lens use a maximum of approximately 5 seconds exposure.

    Use proportionately longer times for shorter focal lengths, shorter
    for longer focal lengths.

    This is something you can do in your head.

    A 50mm lens is approx 1/3rd of 135mm, so you can use 15 seconds.
    A 500mm lens is approx 3x a 135mm, so you can use 2 seconds.


    Barry Smith, Dec 4, 2003
  18. Juan R. Pollo

    DHB Guest

    It's now 3:30am where I am in MA USA & 17.3 deg. F. Just came back in
    from taking 10, 15sec. pictures of the sky & I froze my A** off because my
    body has not yet adjusted to the cold.

    Anyway the 1st 4 were taken with the camera set to ISO 50, 15 sec., f8,
    WB=tungsten, Picture effect=vivid, self timer set to 2 sec. Took 2 shots in
    a row without moving the camera in order to see if what looked like stars
    might have been noise or hot pixels. Now I am a bit perplexed because the
    only dots I noticed in all for pictures was 1 bright star & nothing that
    looked like your picture.

    The next 6 pictures I did exactly the same thing but changed the f stop
    to 2.8 (wide angel wide open) & repositioned the camera lower in the sky in
    order to have part of my neighbor's house in the picture too. Got more
    stars, a little area of white clouds & may neighbor's house was dimly
    visible against the sky. However not a single picture showed a single
    unexplained or expected dot of light.

    Either I got real lucky & got a better than average CCD or I am doing or
    setting something different than you are. As for the "tungsten" WB & the
    vivid modes, I simply forgot to change them from the settings I usually use
    when taking existing light pictures both indoors & outdoors. Probably
    should try resetting the WB to Sunny or cloudy & drop the vivid photo effect
    mode to see it that makes any difference.

    The very cold night may have reduced noise so much as to be undetectable
    but if that was true than I should have seen it at least on the 1st 2 shoots
    as I took them as soon as I got outside before the camera/CCD could have
    cooled down much. So now it all seems like more of a mystery so I'll have
    to do it again at a later date & if I still see nothing I'll try it with my
    other digital cameras also.

    Sorry if this did not help much but I try again another night.

    Right now it's well past my bedtime, so before I fall asleep with my
    head on the keyboard, I best call it a night.

    Respectfully, DHB
    DHB, Dec 4, 2003
  19. Big difference from the mid 70's temperature when my pictures were taken!
    I'm also out on the edge of town where the skies are a bit clearer than
    Can't you see any stars with the naked eye? Down here you can, what I do is
    point the camera to a cluster. The sky was clear when I took my shots.
    I'm not sure if the exif data carried on to the picture I posted on
    Photosig, but everything was set to normal except for the long exposure. I
    did need to re-size the picture fpr Photosig, but no other adjustments were
    It looks like I need to wait for a cold night myself. Thanks for the

    Juan R. Pollo, Dec 4, 2003
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