Beginner Long exposure and blue/green/red dots in dark background

n0chex

n0chex

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Andrew
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Hi all is it dust on the sensor or lens that causes blue/green/red/white dots on long exposures on a canon 50d with a 24-105l.
I have checked the sensor and the lens and both look pretty danm clean if not perfect.

I have tried to show below in a screen capture but not sure if you will be able to see them at 100%
 
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It's either iso sensitivity or stuck sensor pixels (possibly called hot pixels?)...
Should always be in the same places.

If you are lucky, using the sensor clean function in the camera may clear a few. Otherwise, note their positions for later processing.

Some processing allows you to use a mask to filter them out, or if shooting raw, make them less visible...
 
The camera has not done that many shots 2800 ish and they seam to be all over the picture and at a guess 20 but it was a long exposure 30 seconds.
What bothers me is I like night time photography so I guess I am going to have to do a lot more work in removing in software.
 
Your software should be able to map them and filter them out automatically. But you'll need camera and or software specific instructions.
 
If the 50D is similar to the 40D, a couple of shots with long exposure noise reduction turned on and the lens cap left on should map out the hot pixels - but as Phil has said, refer to your manual for the exact details.


This is quite normal, nothing to worry about
 
Ok cool guys will look into this
 
Alastair said:
If the 50D is similar to the 40D, a couple of shots with long exposure noise reduction turned on and the lens cap left on should map out the hot pixels - but as Phil has said, refer to your manual for the exact details.


This is quite normal, nothing to worry about
Click to expand...

Just a note though... you can't use long exposure noise reduction if trying to do something like star trails using a stacking program. Long Exp NR actually takes two exposures. First is the actual exposure, then it will activate the sensor for the same amount of time with the shutter closed to generate a "dark" image.. which is essentially a black frame that will also have generated the same pattern of hot pixels. The camera's processing then replaces those pixels with an adjacent non hot pixel on the finished image by usng the "dark" as a map to where the hot pixels are. The upshot is, that if you used a 1 minute exposure, the camera will be busy for 2 minutes... so star trails will have gaps in them.

There's software out there than can use a dark frame to remove hot pixels. A bit of Googling will sort you out.

BTW... every camera I've owned has hot pixels on long exposures.
 
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