I have a 24/240 mm telephoto lens.
It sends images to a 42mpx full frame, pretty fast sensor.
But even with all that I have had trouble getting any decent pictures of harlequins.
They are really skittish and stay either just at or beyond really good reach of my lens, or, if they are close in on the beach, they fly off the minute they see me.
So this was an unusually lucky shot.