What is this light spot called?

Remember the convex lens course in middle school physics?

Those "light spots" are usually called fuzzy light spots. The principle of formation is that the light emitted by the light-emitting point of an object that is not in the focal plane passes through the lens and is imaged on CCD or cmos, instead of a point, it is a circular spot formed by the deviation of these condensing points from CCD. For details, please refer to the resources, and others have drawing instructions.

When these circular spots are small enough, they are still points to the human eye, so this tolerance (the distance that an object deviates from the plane you are focusing on but does not look "imaginary") is called depth of field. An object far from the depth of field is "blurred" because every point on it becomes a big point.

To achieve this effect, SLR can use a large aperture, DC can be as close as possible to the subject, so that the farther luminous body can be outside the depth of field.