  In the analysis of the on-axis intensity for a highly focused optical field
it is highly desirable to deal with effective relations aimed at characterizing
the field behavior in a rather simple fashion. Here, a novel and adequate
measure for the size of the region where the axial power content mainly
concentrates is proposed on the basis of an uncertainty principle. Accordingly,
a meaningful relationship is provided for both the spread of the incident beam
at the entrance of the highly focused optical system and the size of the region
where the on-axis power mainly concentrates.
