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  1. #1
    Meat Shield is offline I'm new here!
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    Default Is my sensor going bad?

    Hi everyone - long time reader, first time poster.

    I have a Rebel XTi, love it, using it for four years or so. A few nights ago I took some shots of the moon, and noticed a red spot (looks to be a couple pixels in size) showing up against the black of space. Best seen here: Swoosh on Flickr - Photo Sharing!, go to full size and look directly under the moon.

    I have taken moon shots before under similar conditions and never seen this, so it is a recent development. Any ideas? Sensor starting to fail? And if so, can I do anything to prevent it?

    Thanks in advance!

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    CelticOrigins is offline For the love of it.
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    Hi, looked at your pic on my iPhone and couldn't see the red pixels but as far a I know they probebly Hot Pixels caused by the long exposure, did you have long exp. Noise reduction on? If they are Hot Pixels then it happens to all digital cameras as far I know.
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    OsmosisStudios's Avatar
    OsmosisStudios is offline Don't Panic
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    The red band along where the moon meets the black of the sky? That's probably Chromatic Abberation.

    The lens you used (Canon EF 75-300 f/4-5.6) is known for some rough CA and distortion at 300mm and f/5.6 (wide open). That would be my guess. Did you shoot RAW? If so, you should be able to remove it.
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    BCampbell is offline verb noun
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    I see the red spot just below the moon, and a blue one just above and to the right of it.

    This wasn't a very long exposure so it's not likely hot pixels due to sensor heat buildup. However, as they age, sensors do often pick up hot/dead pixels. Some editing software and in-camera noise reduction can compensate for this.

    If you want to test this, put the lenscap on and take a 1/160th exposure and see if those same spots show up.

    Usually very dark fields will emphasize dead/hot pixels, and you won't notice them on "normal" shots, so you don't have to worry. But if you have a persistent hot pixel, you can look into setting your software up to automatically correct your photos for it (since they show up in exactly the same place every time, it's pretty simple to automate).

    In my test of UFRaw 0.17 I have an example of what hot pixels on a dark, long exposure looks like. The photo those examples were taken from was a 15 second exposure.

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    Meat Shield is offline I'm new here!
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    No, it wasn't a long exposure, only 1/160. And no, its below the chromatic abberation.

    Go to the original size, center the moon, and move down half again the height of the moon. You should see a small red spot in teh middle of the black.

    It is also showing up on multiple shots from that night, so it was not just that picture.

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    Meat Shield is offline I'm new here!
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    Quote Originally Posted by BCampbell View Post
    Usually very dark fields will emphasize dead/hot pixels, and you won't notice them on "normal" shots, so you don't have to worry. But if you have a persistent hot pixel, you can look into setting your software up to automatically correct your photos for it (since they show up in exactly the same place every time, it's pretty simple to automate).

    In my test of UFRaw 0.17 I have an example of what hot pixels on a dark, long exposure looks like. The photo those examples were taken from was a 15 second exposure.
    Thanks for the link. I'll take a look at that.

    To ease my soul, is this a sign of imminent failure, or just something that I have to worry about only when looking at a black field and zoomed in all the way, with no other bad spots ever potentially?

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    BCampbell is offline verb noun
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    Quote Originally Posted by Meat Shield View Post
    To ease my soul, is this a sign of imminent failure, or just something that I have to worry about only when looking at a black field and zoomed in all the way, with no other bad spots ever potentially?
    Well, it depends.

    Generally, hot/dead pixels happen on an individual basis. It doesn't mean your entire sensor is going to fail. And in fact, you can send your camera in to Canon to get it "repaired". The thing is the repair is just doing to the camera's processor what you would do in software: detecting where the hot/dead pixel is and telling the processor to interpolate the "correct" information from the pixels around it.

    In fact, every sensor in every one of our camera is "born" with bad pixels! They are simply calibrated at the factory to interpolate those bad pixels out so we never notice them.

    Keep an eye on it and see if you notice it in any other pictures. I doubt it just started happening on this shoot, so you could even go back and look at other more recent images. Most likely you've just never noticed it, and it probably won't show up or be an issue at any other time. Just file it away and look for it over the course of your next few shoots.

    And, as this is a sensor issue, it doesn't have anything to do with your focal length.

    But no, this isn't a sign of any imminent catastrophic failure.
    Last edited by BCampbell; 04-23-2010 at 02:43 PM.

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    Meat Shield is offline I'm new here!
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    Whew! on the 'no immenent failure'. Thanks!

    (also, when I said zoomed all the way in, I meant on my monitor, not my lens!)

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    BCampbell is offline verb noun
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    Quote Originally Posted by Meat Shield View Post
    (also, when I said zoomed all the way in, I meant on my monitor, not my lens!)
    Oh right, yeah, you'll notice this stuff at 100% and sometimes 50%, but just like noise is less noticeable as we resize images down, how the software scales images to smaller sizes can often hide hot pixels.

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    andrecb is offline dPS Forum Member
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    I dont think that is a hot pixel, it is too out of focus.

    Either its dust in sensor, or you captured a star or planet, or just noise.
    reposition the moon to the left or right and see if it appears

    P.S. there is also a blue one in the up right corner of the moon. out of focus too. I dont think you have to worry about that.

    try ISO 100 F11
    Last edited by andrecb; 04-27-2010 at 09:22 AM.
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