As a kid, I used to love playing with magnifying glasses and trying to start small fires. I’d hold up the magnifying glass above the pavement, get the angle just right, and then focus the laser beam of light on a leaf (or poor unsuspecting ant) for a few seconds before it started smoking.
If the angle of the magnifying glass was slightly off, or the distance from the pavement was not right, the beam of light would be a large unfocused, lukewarm circle and completely ineffective.