Strange soft-spot on shots

[adrjork]

Hi everyone,

I've just noticed a strange black little soft-spot on my recent shots (see attached image). I say "strange" because it's always on the same position in the frame, but NOT on every shots!
Naturally I thought it was same dirt on one lens, but it's not, because it's visible on shots made with different lenses.
So I thought it was dirt on sensor, instead, but - as I said - it is visible NOT on every shots.
The only one solution I can give (but I'm NOT an expert) is that it's a little grain of something (dirt, ash...) in short something "three-dimensional", and sometimes, when light has a peculiar angle, I can see its shadow...

Just 2 questions:
1. What is it in your opinion?
2. How can I solve? (Service?)

Thanks a lot.

[Walter Schulz]

Looks like dirt on the sensor.
There are lots of tutorials on the web how to detect sensor dust/dirt and how to get rid off it.

[adrjork]

Thanks so much

[a1ex]

For correcting existing footage, taking one or more pictures (video frames) of a white wall (i.e. a flat field) should do the trick, assuming you have not cleaned the sensor yet. You will need to use the same aperture and (roughly) the same focus distance when capturing the calibration frames, as these spots seem to be the shadow of dust particles on the sensor (i.e. more obvious at f/22 and the like). To avoid capturing the wall texture, make sure the image is out of focus and/or move the camera around and average all calibration frames.

Check these notes (from here) for another defect corrected with the same technique.

@Danne actually experimented with this, so he may have better advice.


Edit: found some similar artifacts in a H.264 clip recorded at F/22. Here's how I've fixed them:

flat-h264.html (10MB; one artifact still visible)
flat-h264-advanced.html (23MB; completely fixed)