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Thread: Mysterious Red Dots

  1. #1

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    Mysterious Red Dots

    Hello,

    I was out over 4th July and tried my luck on some firework pictures. When I got back home I noticed some bright red dots in the image. First I though my monitor had developed some dead pixels, but after some investigating I found these dots on all of my most recent pictures. I ruled out my lenses, because they appear on either lens with either focal length.
    My guess is that there is something going on with my camera sensor. Can this be fixed?

    Here is an example picture. Warning, I didn't resize the picture, so it could be a long wait for you modem-people

    Thanks,

    Kirk

  2. #2

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    Re: Mysterious Red Dots

    Quote Originally Posted by Kirk View Post
    My guess is that there is something going on with my camera sensor. Can
    They're just a few "hot pixels" - every camera has a few, but some software removes them automatically. They're usually only visable with long-exposures.

    I believe that Canon can map them out, but I've never had it done personally.

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    Re: Mysterious Red Dots

    Nikon cameras can do a long exposure noise reduction to reduce this as the hot pixels tend to happen in the same place each time. What the camera actually does, is reshoot the image but without opening the shutter. The resulting image, supposedly totally black, is then subtracted from your real image. This cancels out the hot pixels almost 100%. The downside is that the hot pixels get more widespread when the sensor is warmer, and the sensor will heat up slightly during the shot. The "dark frame" as its know, therefore has slightly more hot pixel noise so the subtraction takes away slightly more than it ought to.

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    Re: Mysterious Red Dots

    I'm not sure if those are hot pixels. Hot pixels are normally by themselfs and not gathered in groups.

    I would guess lens flare or UFOs.

    Try to take a picture with your lens cap on with the same exposure. If those red dots show up still, you know they are hot pixels. If not, they are little green men flying space ships.

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    Re: Mysterious Red Dots

    Quote Originally Posted by Raycer View Post
    I'm not sure if those are hot pixels. Hot pixels are normally by themselfs and not gathered in groups.

    I would guess lens flare or UFOs.

    Try to take a picture with your lens cap on with the same exposure. If those red dots show up still, you know they are hot pixels. If not, they are little green men flying space ships.
    Unfortunatly, extraterrestrial life will remain a mysterie.

    I took a picture with the lens cap on, and the dots are still there. Actually, I found a couple more but these are white..
    Is that normal for digital cameras? I have never seen that before (or maybe I just didn't pay close enough attention), so it comes as a suprise to me. Will this "problem" get worse over time, or is it just some imperfections during manufacture?

    Thanks for the help.

    Kirk

  6. #6
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    Re: Mysterious Red Dots

    shouldn't get worse over time but it's unavoidable due to silicon manufacturing and the amount that appear of sensors that pass QT are always negligable. Often they are only noticable in certain scenes and exposure lengths. I'd guess due to tighter control on high end dslr's you'd get less on one of those at say 15seconds than you would on a cheaper point an shoot (hence shutter speeds are capped off on cheap cameras since hot pixels become more of an issue on long exposures).

    Basically the defects in the silicon making up the photo receptors means they detect light at a higher rate than they should for the actual amount of light hitting them (or register light even in the abscence of light like when lens cap is on). It's very unusual to get ones grouped into same tight spot (prob wouldn't pass qt) so easily colned out and missing a handful of accurate pixels is no problem. A lot of developing software automatically removes them, or if not you can use something like "blackframe NR" which is free and made for specifically removing hot pixels.

    Some increase at a faster rate than others (in the absence of any light) so are less noticable except on really long exposure.

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    Re: Mysterious Red Dots

    sorry to hear that... especially to see them in clusters like that. But 13" exposure is pretty tough on the sensors.

    Not sure what Canon's policy is on hot pixels. Nikon call them defect pixels. If they show up in shots 1/4" or faster, then Nikon will repair the camera... or at least map them and hide them. You might want to check this with Canon.

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    Re: Mysterious Red Dots

    Kirk,

    Out of interest, what software are you using for post-processing? I mostly use Photoshop CS3 and it just removes them automatically.

  9. #9

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    Re: Mysterious Red Dots

    Hello,

    I had a very similar looking problem with a Canon 400D. It developed a problem when it was about a year old whereby it made obvious red dots. I tried taking a couple of pictures with the lens cap on, and I got the same red dots. I could also see pale grey patches (which were not visible in an ordinary picture). The dots were not typical long exposure hot pixels. They occurred in short exposures as well. After some digging around I found the following remedy.

    With a lens fitted to the camera and the lens cap on, activate manual sensor cleaning, hold the camera with the lens pointing more or less down (I'm not sure how important the orientation really is), and keep it like that for about half a minute. Then turn the camera off. That cleared up the problem in my case - both the red dots and the grey patches. I read several reports of a similar procedure working for other people, although I did also see one complaint by someone that it made things worse.

    I don't know what was going on, but my guess is that regions of charge built up on the sensor, or charged particles stuck to the hardware near it, and this procedure encouraged them to dissipate or dislodge. That's just a guess though.

    I hope this helps,
    Will

  10. #10

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    Re: Mysterious Red Dots

    With a lens fitted to the camera and the lens cap on, activate manual sensor cleaning, hold the camera with the lens pointing more or less down (I'm not sure how important the orientation really is), and keep it like that for about half a minute. Then turn the camera off. That cleared up the problem in my case - both the red dots and the grey patches. I read several reports of a similar procedure working for other people, although I did also see one complaint by someone that it made things worse.
    HA, that fixed it!
    I still have one red dot left, but I will try your trick a couple more times.
    Thanks for that tip!

    Kirk,

    Out of interest, what software are you using for post-processing? I mostly use Photoshop CS3 and it just removes them automatically.
    I use Photoshop CS4. I hadn't actually loaded the pictures into PS before posting here. I just checked it out and it indeed removes the dots.

    Thanks to both of you for your help!

    Kirk

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