The resolution of the image generated by the ultrasound beam is not the same across all depths. The sound waves emitted from the crystal array in the probe can be focussed electronically at a level of your choosing, which you should adjust to the level of the organ of interest. This maximizes the resolution of the image at that depth. You should alter the focus level continuously during scanning as your target organ changes in depth.

When using ultrasound to guide needles in patients (for example when inserting a central line), set your focus at the level of your needle and not at the level of the target vessel. This will help you see the position of your needle more clearly.