Green spots on night photos

Discussion in 'Nikon' started by joel_tan|1, Nov 25, 2006.

  1. Hi,

    I was hoping to get some advice on green spots in my photos. I'm a relative
    beginner and just got myself a Nikon D80 recently. Was experimenting with night
    photos and found green spots streaked across one of my photo (shot with
    50mm/1.4D, f/2, 1 sec, ISO400). However, I retook the same photo at f/11, 30sec,
    and there were no such green spots.

    f/2, 1 sec exposure (hv circled the areas with the green spots in white):
    <http://www.medioluna.com/images/f2-1sec.jpg>

    f/11, 30 sec exposure (downsized the image because no real problem, just for
    comparison's sake): <http://www.medioluna.com/images/f11-30sec.jpg>

    Does anyone know what the problem is? When I examine the latter photo in greater
    detail, I can spot similar aberrations, except they're not so obvious so I can
    live with them. However, those in the earlier exposure were very glaring. Is
    this a problem with the lens or the camera? What can be done to fix it?

    Hope for some advice. Thanks!
     
  2. Hi Joel,

    My first thought was airplane, helicopter, ufo (???) flying through the photo but if they are showing up in the other one too I'm not sure.

    Mainly, I wanted to let you know to check your links; the > is being included when you click on it so it's causing an error in the posting.

    Cheers,

    -Rob
     
  3. Thanks Rob.

    I actually took two photos at f/2, 1 sec, one after another. They both show the same green spots in the same places, so I don't think it was an airplane.

    What I'm wondering is why: I read about dead/hot pixels, but these tended to occur at longer exposures. However, in my case, it's not occurring at the longer exposure (30 sec), but at the shorter one.

    With regards to the links, I can't figure out how to edit my original post, so if it's any help I'll re-post the links here:

    for the f2, 1 sec img: http://www.medioluna.com/images/f2-1sec.jpg

    for the f11, 30 sec img: http://www.medioluna.com/images/f11-30sec.jpg

    Thanks!
     
  4. I'd be willing to bet that it's internal reflections in your lens at F2 and that it's green due to the coatings on your lens elements. If that's the case then there's nothing you can do but shoot at a higher aperture in these sort of shots.

    At F2 you're using more of your glass in your lens while at F11 you're only using a small portion in the center of your glass. Using more glass (F2) allows more opportunity for light to bounce around.

    -Tom
     
  5. Ghosts are in your f2 shot. the reason why they're not in your long exposure shot is that you stopped your lens down to f11 which obviously eliminated the ghosts. Did you crop the image any? The ghosts in the f2 shot should show reciprocally in frame ie.. top left ghosts are being caused by lower right lights.
     
  6. It looks like an effect due to lens filter. Did you have a UV or protection filter on? You have got to take it off for night shots.
     
  7. I agree, this is probably a filter artifact. I observed this with my D70s. After removing the UV filter it did not occur anymore.
     
  8. Definitely ghosts of the bright lights mear the bottom of the images. Note that they are 180 degrees from the parent lights. As mentioned, they are caused by reflections from the film or sensor returning back through, and collimated by the lens, reflected by the flat filter faces, and reimaged by the lens 180 degrees. Large apertures also increase the brightness of the ghosts by allowing a greater percentage of the light to be reimaged. If you must use a filter, get one that is multi-coated.
     

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