What are these colored specks?

Discussion in 'Digital Photography' started by Caliber26, Aug 1, 2013.

  1. Caliber26 macrumors 68000

    Caliber26

    Joined:
    Sep 25, 2009
    Location:
    Orlando, FL
    #1
    I've noticed these specks on some of my images and, at first, thought it was noise but I'm shooting with the lowest possible ISO and I'm not adding any exposure to the images in post processing and they're still present. I hadn't seen them before. Are they coming from the sensor or the lens? Does anyone know what they are and how to avoid/fix that?

    Thanks!

    I had to bump up the exposure for the purpose of being able to show the colored specks I'm referring to:
     

    Attached Files:

  2. GoCubsGo macrumors Nehalem

    GoCubsGo

    Joined:
    Feb 19, 2005
    #2
    Seems like an issue with the sensor. I'd take it in to have it looked at.
     
  3. InTheMist macrumors member

    InTheMist

    Joined:
    Jun 22, 2013
    #3
    Do you notice them in long exposures perhaps worse?
     
  4. Caliber26 thread starter macrumors 68000

    Caliber26

    Joined:
    Sep 25, 2009
    Location:
    Orlando, FL
    #4
    All of my pictures are at night so they're all taken with exposures of 15 seconds or more. I'll have to test it out on with a fast shutter and see if the specks are still present. They're only visible against darker parts of the image and what's odd it's that they're in some pictures and not in others (from the same session). I checked the settings for each image and they're all about the same, so I can't understand why it alternates.
     
  5. TheReef macrumors 68000

    TheReef

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    Sep 30, 2007
    Location:
    NSW, Australia.
    #5
    Looks like hot pixels, caused when the sensor heats up too much during a long exposure. Some sensors are more prone to this than others.

    Try upgrading your camera's firmware, sometimes the camera software can mask them out.
     
  6. Caliber26 thread starter macrumors 68000

    Caliber26

    Joined:
    Sep 25, 2009
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    Orlando, FL
    #6
    Thank you! I'll give this a shot.
     
  7. InTheMist, Aug 1, 2013
    Last edited: Aug 1, 2013

    InTheMist macrumors member

    InTheMist

    Joined:
    Jun 22, 2013
    #7
    Yeah, definitely seems like hot pixels. Most cameras have a feature called something like "Long Exposure Noise Redution" where it takes a second exposure with the shutter closed that was as long as the first, compares the two images and digitally removes the hot spots from the original image.

    It comes from the fact that sensors get hot with long exposures, and the firmeware feature will virtually eliminate it. In Nikon land it's called "Long Exposure NR".
     
  8. Laird Knox macrumors 68000

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    Jun 18, 2010
    #8
    What was the air temperature? You can get a lot of extra sprinkles on warmer nights.
     
  9. snberk103 macrumors 603

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    Oct 22, 2007
    Location:
    An Island in the Salish Sea
    #9
    Compare several images under different conditions.... but where the conditions create similar exposures. Are the specks in the same place in each image. If so, probably bad sensor pixels. If their distribution is random, then it's likely just noise.
     
  10. Borntorun macrumors member

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    Nov 15, 2011
    Location:
    Perth, Australia
    #10
    You said you are shooting at lowest ISO - I have seen these specs crop in at ISO50 on my 5dmkii, try shooting at ISO100 as a minimum - and see if it disappears.
     
  11. Caliber26 thread starter macrumors 68000

    Caliber26

    Joined:
    Sep 25, 2009
    Location:
    Orlando, FL
    #11
    Good point. I had actually turned this feature off on my D7100 a few weeks ago, when shooting 4th of July fireworks, because I didn't want to miss shots in between bursts while it performed the NR. I'm going to upgrade the firmware and turn the feature back on and see if that makes a difference. Thanks!
    (question aside: I've been told the LENR feature is pointless if I'm shooting RAW and that I can leave it off unless I'm shooting JPEG. Is this true?)

    ----------

    The specks seem to randomly move around from image to image and only visible against dark backgrounds. They are completely invisible against anything white. Plus I'm not doing anything that would warrant noise and I've intentionally added noise, to compare the specks to the noise, and they're very different looking. I'm going to turn on the Long Exposure Noise Reduction feature (on the camera) back on and see if that makes a difference.

    My Nikon D7100 only goes down to ISO of 100.
     
  12. Laird Knox macrumors 68000

    Joined:
    Jun 18, 2010
    #12
    If the spots move then it is likely just sensor noise. Take a picture with the lens cap on in the same conditions with the same settings, then again when the camera has been sitting in your air conditioned home for a couple of hours. Does the second picture have fewer hot pixels?

    If you are doing a lot of exposures (like the fireworks) you can take a dark frame exposure before and after he show. Then in Photoshop you use the dark frame to subtract the bad pixels and hot spots. This is basically what the long exposure noise reduction does. This kind of noise does not drastically change from shot to shot so you don't really need to take a dark frame every shot.

    The D7100 should be pretty good in most circumstances. Long exposure noise reduction isn't typically required. Contrast this with the D70 and its CCD sensor and you had to basically do it every long exposure.

    Bottom line is you will always have some hot pixels in a dark image. It is the nature of the beast. With millions of photo sites you will have one that is bad, gets hit by a stray cosmic ray or simply decides to take an early weekend. Think of removing hot pixels in post the same way that you have to remove dust off of film prints. It just happens.
     
  13. ChristianJapan macrumors 601

    ChristianJapan

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    May 10, 2010
    Location:
    日本
    #13
    Is that 100 lowest native value ? Or an artificial one like the 50 from Canon ? I remember once read that Nikon lowest native ISO is 200 but that can be wrong and/or outdated.
     
  14. Laird Knox macrumors 68000

    Joined:
    Jun 18, 2010
    #14
    Yeah, in recent years the Nikon's hit 100 before going to LO1. My D70 and D60 bottomed out at 200, and my D7000 and D800 go to 100.
     

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