Image sensor defect?

Fish in a Barrel · Jan 9, 2012

Tonight I'm shooting some long exposure shots of the sky (5-15 min) with my XSi. I've noticed something disappointing. In each shot there are scattered bright dots, blue and red, throughout the image. They are in the same place in each shot, and they don't have any motion blur, so they're not an aircraft or a star. I've never seen these in an image before, but I've never taken such a long exposure before.

Does this sound like a defect in the camera, or is there something going on here that I'm not appreciating?

(Yeah, I know a sample image or two would help, I'll post when the last shot finishes.)

MissCream · Jan 9, 2012

Hot pixels or noise is my guess.

Fish in a Barrel · Jan 9, 2012

MissCream said:
Hot pixels or noise is my guess.

Stuck pixels was my guess as well. There are quite a lot of them, though; I'd guess around 20. Is this common? My buddy's new T3i didn't seem to have any.

I uploaded some example shots, but picasa recompressed the image to the point that the hot pixels disappeared. I guess that's one way to address the problem.

marmots · Jan 9, 2012

it was probably just long exposure noise

iresq · Jan 9, 2012

Can you reproduce with a short exposure? I'm guessing over heat as well.

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MLeeK · Jan 9, 2012

Can you post for us to see?

D.Page · Jan 10, 2012

They are 'stuck' or 'hot' pixels. There is an 'undocumented' fix for this, which Canon don't tell you about. It does not work with all Canon DSLRs, mainly the latest models only. Just give it a go and see if it clears them.

With a lens attached and its lens cap on, put the camera into the manual cleaning mode, so that the mirror flips up, as if you were going to give the sensor a clean. Just leave it in this mode for about 30-60 seconds. Turn the camera off. Your camera should no longer have the 'stuck' or 'hot' pixels. What the camera does this is that it 'remaps' the sensor, concealing any 'stuck' or 'hot' pixels present on the sensor so you won't see them anymore in any of your subsequent pictures. You may have to carry the procedure out a few times for it to work.