Blur is not as simple as 'out of focus'.
It can be created in many ways, some deliberate, for special effect.
► Smear on a lens (sometime as special effect, artistic vignette blur)
► Hot hair rising (from a road surface by example)
► Thick dust in the air
► Weather conditions
► Antialiasing filters in older cameras (a feature to avoid moiré)
Basically anything that prevents a camera to capture a scene with sharp details
It can be created by almost anything such as a dirty lens, weather conditions, dust, heat rising from a surface...
Then you have blur created by the camera as not being held steadily (motion blur).
Blur can can be created deliberately in order to create a special effect seen most often in images of moving objects such as water, cars...
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