is always a certain amount of dust floating around in the air. You may have noticed
this at the movies when you look up at the light coming from the movie projector
and notice the bright sparks floating around in the beam.
In the same way, there are always dust particles floating around nearby when
you take pictures with your camera.
When you use the flash, the light from the flash reflects off the dust particles
and is sometimes captured in your shot.
Of course, dust particles very close to the camera are blurred since they are
not in focus, but because they reflect the light more strongly than the more
distant main subject of the shot, that reflected light can sometimes be captured
by the camera and recorded on the resulting image as round white spots. So these
dots are the blurred images of dust particles.
You can reproduce this problem relatively easily by taking a picture right after
you put away goods that create a lot of dust, such as feather bedding.
In actual photography, this problem frequently arises in shots taken at construction
sites, etc. It may also occur when it is raining or snowing. Compact cameras
in which the flash and the lens are close together are particularly susceptible
to this problem.