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.

Compression or shear

Worked example

Elements in the active group	16
Element pitch	0.6 mm
Passive aperture (elevation)	10 mm
Probe frequency	5 MHz
Wave mode	shear
Material velocity	3240 m/s
Focal distance (sound path)	30 mm
Near field length	51 mm
Normalised focal depth F/N	0.59
−6 dB focal zone length	27.27 mm
Approximate start of the focal zone	16.36 mm
Approximate end of the focal zone	43.64 mm

N = 51.003 mm and F = 30 mm give S_F = 0.5882. FZ = 51.003 × 0.5882² × 2/(1 + 0.2941) = 51.003 × 0.34598 × 1.54548 = 27.27 mm, so the zone runs roughly 30 − 13.64 = 16.36 mm to 30 + 13.64 = 43.64 mm.