Camera, Lens or both

[jamisan]

Ok, I sent my Rebel XS in (out of warranty) to have the image sensor cleaned (dust spots all over images)
Got it back and all semed well, or so I thought. I took several night time shots of a prairie fire several miles away. Using a tripod, shooting RAW,around 5 sec exposures. I looked at the images and there are red,green and blue dots all over the image. ISO was around 400. Ok I was thinking the image sensor may be bad. Canon wanted over $200 to fix so I just bought a Rebel XSi from Canon refurbed. Was playing around the other day and took several shots with the lens cap on. Sure enough red,green and blue dots. The higher the ISO the worse the dots. I then took several shots with all three of my lenses and got the same results
So, is the image sensor bad on both cameras, are all my lenses bad or is this just an inherant problem with canon Rebels

[Bob Atkins]

It's an inherent problem with all digital cameras. It's digital noise and it shows up most on long exposures. Very bright pixels are known as "Hot Pixels", but all cameras will show some low level color noise (chroma noise).

In general you see lower levels of noise with larger pixels, all else being equal. Better (or stronger) noise reduction techniques can lower it somewhat, but you'll see it with every digital camera especially at higher ISO settings and with longer exposures.

[KeithB]

You can try using the "dust delete data" technique to remove them if they show up against a white background.

Or you can use the custom function that creates a second exposure with the shutter closed that then subtracts the hot pixels.

[jamisan]

Bob,
 I know about the "noise" with long exposures, I had just never seen the red,green,and blue dots before. Altho they aren't very bright but they are noticable and with the ISO on 100 you cant see them at all.
Keith, They do not show up on white background just black or very dark. I did enable the custom fuction haven't tried it yet but makes sense

Thanks guys

[Bob Atkins]

Sounds like you have some "warm" pixels. If they always show up in exactly the same positions, then they are warm/hot pixels. If their position changes from exposure to exposure then they are just statistical noise.

There are programs which can be used to map out hot pixels. They interpolate values from near by pixels.

These pixels have slighly more gain/sensitivity/leakage than average, so they tend to light up brighter than average in longer exposures.

[whizkid]

You can see samoles of chroma noise in camera tests conducted at DP Review They do both raw and jpeg noise assessment using all ISOs of the camera as well as luminance noise in black and grey tests. Not of long exposures but it  demonstrates much of what Bob was pointing towards in his reference to chroma noise..