How to Build a Laser Death Ray
Depth Of Field
The figures below show a death ray focused at two different distances. In the top figure, the ray is focused twice as far away as in the bottom figure. Suppose the light has to be concentrated enough to fit inside the small vertical red bars in order to cause damage.
Notice what happens - when the death ray is focused farther away it has a greater distance between the red bars. It can cause damage over a wider range. If it had to bore a hole in its target, for example, it could bore a deeper hole when it is focused farther away. This range over which the death ray can cause damage is called the depth of field.
If you need to drill deeper into a target than your depth of field, you might be able to sweep the focus of your beam further into your target - but this requires knowing how deeply you have already drilled. Another option is to reduce the initial diameter of the beam, as shown in the image below, which is focused to the same distance as the death ray of the lower of the two previous images, but since it starts narrower it has a larger depth of field.
Note, however, that the beam cannot start out too narrow, or it will damage the optics that focus the death ray.
Blaster beams that propagate through an atmosphere can use self focusing and filamentation effects to increase their depth of field.