| I use a Canon 7D. On several occasions, when taking photographs at
| relative high ISO (e.g. 1600 or 2000), I have noticed red dots on the
| image. See this image:
[...]
| I have 2 questions:
| 1) What causes this?
I believe that these are usually caused by 'hot pixels' on the sensor.
They appear mostly at higher ISOs because high ISOs magnify the effect
of the pixels having more electrons than they should. Having some hot
pixels is apparently common, where they are can change over time, and
they don't always happen in red pixels (although red pixels make them
the most obvious).
| 2) Is there an easy way in dt to remove it?
Darktable has a 'hot pixel' module that's supposed to deal with them.
I've never tried it myself, but you could give it a spin and see how
well it works for you. There are more advanced techniques like 'dark
frame subtraction' (where you take a completely black frame at the ISO
you're going to use and the software basically subtracts it from your
real RAW), but I don't think Darktable supports them.
- cks
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