Depth of field of a focused beam

A focus is not a point but a cigar shaped zone. Its axial length, the depth of field or focal zone, follows FZ = N · S_F² · 2/(1 + 0.5·S_F), where S_F = F/N is the focal depth normalised to the near field length. The relationship is strongly non-linear: a tight focus at S_F = 0.3 gives a short zone, while at S_F = 0.8 the zone stretches over most of the near field.

The practical consequence is that one focal law only inspects a band of thickness. Outside the focal zone the beam is wider and the amplitude response of a given reflector changes, so amplitude based sizing drifts. Covering a thick section therefore needs either several focal depths, an unfocused law with TCG, or a depth focus law set that steps the focus through the wall.

The zone is not quite symmetric about the focus - it extends slightly further towards the probe than beyond it - so the limits given here are a working approximation. Where focal zone boundaries matter, confirm them on side drilled holes at the depths concerned.

S_F = F / N
FZ = N · S_F² · 2 / (1 + 0.5·S_F)
zone ≈ F ± FZ/2