Final answer:
The incorrect phrase is 'a bigger focal zone'; focusing results in a smaller focal zone, leading to better near zone resolution and a shorter near zone with potentially less lateral resolution in the far zone.
Step-by-step explanation:
The phrase that does NOT describe an effect that focusing has on the ultrasound beam is 'a bigger focal zone'. In fact, focusing the ultrasound beam results in a smaller, more concentrated focal zone, which leads to enhanced resolution in the near zone. As the numerical aperture (NA) increases, this focal zone becomes more condensed, providing a sharper image and allowing for better resolution of objects. However, focusing may also result in a diminished lateral resolution of the far zone, because the beam diverges after the focal zone, causing the resolution to decrease as the distance from the focal point increases. This convergence of the beam to a focal spot also leads to a shorter near zone, which can be beneficial for imaging applications that require high details at close range.