Artifacts appearing on night photos?

Discussion in 'Photography Beginners' Forum' started by SPL Tech1, Sep 19, 2008.

  1. SPL Tech1

    SPL Tech1 TPF Noob!

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    So I went out tonight to try my hand at some more night photography. Well I found I am getting two different types of artifacts in some of my pictures. The first is a bright blue dot somewhere in the picture. I zoomed in at 800% in some of the pictures displaying this bright blue dot and found there are actually more of these blue dots visible throughout the picture when the picture is viewed under magnification. The second “artifact” appears to be some lens flair of some type. I put my hands around the lens to ensure it wasn’t an actual lens flair and doing so did not correct the problem. Does anyone have an idea where these blue dots and green lines are coming from? When I look at the image and compare it to the actual object I am finding these blue dots are being produced where a light source is on the object. However there are many lights on these buildings and most of them don’t turn blue in the picture.

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    Close up view of the above pic:


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    I found I can get rid of these green line artifacts by increasing the aperture and shutter speed.

    f11 with 30 second shutter:

    [​IMG]


    f5.6 with 8 second shutter:


    [​IMG]
     
  2. SPL Tech1

    SPL Tech1 TPF Noob!

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    [​IMG]
     
  3. ann

    ann No longer a newbie, moving up!

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    the small "dots" may be hot spots created from the longer exposures, and are quite common with these types of expsoures.

    i don't know about the green jagged oubject, i am sure some one else will chime in with a much more indepth answer.
     
  4. DeadEye

    DeadEye TPF Supporters Supporting Member

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    Hot spots and flair. Anytime a light shines directly at a lens you get flair. A hood or in your case hands will block side stuff from flairing but the straight on will still flair. Another trick to reducing flair at night is flagging the camera from near by light.
     
  5. saltface

    saltface TPF Noob!

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    Those aren't artifacts. They're physical manifestations of dead people.
    Do you use Photoshop? There's a tool specifically made for this: spot healing.
     
  6. andrew99

    andrew99 TPF Noob!

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    :lmao:
     
  7. manaheim

    manaheim Jedi Bunnywabbit Staff Member Supporting Member

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    This explains more about noise (of all kinds) than you probably want to know.

    http://www.ronbigelow.com/articles/noise-1/noise-1.htm

    :)

    Basically, in your case, those little dots result from pumping voltage to the sensor for long periods of time, resulting in heat-related color anomolies. It's pretty common, though I get it a ton in my old D100 and get it basically not at all with my D300... not quite sure why, never bothered to figure it out...

    Anyway...

    The healing tool does work wonders on these in most situations. You can also clone-stamp them out of the skies and whatnot. Just zoom into 100% so you know its not doing anything weird to the image.

    Also be aware that some cameras have long-exposure noise reduction where they will basically take two shots- one with the shutter closed (so its taking a long exposure of blackness) and one with it open... it then removes the hotspots from the image. it's not perfect, but it helps. It also burns your batteries 2x as fast and works your camera more, so be aware.

    As stated, that other effect was lens flare. Just clone it out. :)

    Night photography is kinda like this. The key is to get the source data exposed (actually, lean towards overexposed) and then come back and correct your source data. The camera has a heck of a time properly interpreting these scenes. Oh, btw... be sure to shoot night shots in RAW. Color correctness, exposure, etc. are all critical in these, and JPEG will NOT cut it.
     
  8. SPL Tech1

    SPL Tech1 TPF Noob!

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    If it’s only lens flair then why does increasing the shutter time and increasing the f value get rid of the flair as shown in the above pics?
     
  9. mimstrel

    mimstrel TPF Noob!

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    I'm no expert by any means, but could it be related to ISO? at high ISO you get lots of noise, then you make a longer shutter speed and your camera defaults to a lower ISO?

    If not, then I second the ghost theory as stated above.
     
  10. manaheim

    manaheim Jedi Bunnywabbit Staff Member Supporting Member

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    Maybe Helen would know for sure, but I assume changing the aperature is going to change the angle of the light coming into the camera, which would severely affect things like lens flare.

    It is undoubtably lens flare.
     
  11. Alex_B

    Alex_B No longer a newbie, moving up!

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    - The tiny pixel-sized spots in colours like green, blue, red, are hot pixels which you realise in longer exposures and dark backgrounds. Many RAW converter and many cameras can filter them out on the fly. They are not related to high-ISO noise

    - The one large white blue dot which you marked, that is a blue-ish lamp which was in the scene, but you did not realise it unmagnified through the viewfinder. It is not an artefact but real.

    - The green wobbly thing is stray light of some kind, not sure if it is flare, maybe. depends on your lens, angles to other light sources and so on. Some sort of flare can be reduced by smaller apertures as the smaller aperture reduces the light path and might block out those paths which caused the flare.
     
  12. Dragonfly..shotz

    Dragonfly..shotz TPF Noob!

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    I had exactly the same problem last night, four red "hot pixels" in the photo. I took four shots and the hot pixels are in exacly the same spot. Is that common? I had been worried about it till I found this post so thanks heaps! It was a 30sec exposure, ISO 800 F/16 on a canon 40D shooting a storm.

    Very interesting, I just opened the raw file with the converter and then opened image in photoshop and they were gone Alex. Thanks.
     
    Last edited: Sep 20, 2008

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