In ultrasound: When using an annular array for electronic focussing, a greater time delay between energising annular rings results in shorter focal length.
Taken to its extreme, for a focal length of just greater than 0, the time delay would need to be long enough for the ultrasound wave produced on the outer element to reach a point just in front of the centre of the transducer. This would mean that ultrasound waves produced from inner element rings would reinforce the initial wave almost immediately after they leave the probe.
True