Use manual focus, not autofocus. For lightning, set focus manually near infinity/hyperfocal and then avoid touching the focus ring. With a wide lens, stopping down the aperture increases depth of field and makes focus less critical; for nearby strikes, a narrow aperture such as f/5.6 or smaller can help a lot, and some shooters go very narrow for close lightning. If your lens scale isn’t trustworthy, test it beforehand so you know where true distant focus is.

Also, the “bright white bubbles” may not be focus alone. Camera shake from wind, tripod vibration, or even thunder shock can blur the thinner branches of a bolt. Stabilize the tripod well—adding weight can help.

A practical approach is: pre-focus manually before the storm or on a distant object when possible, stop down for more depth of field, keep ISO low, and use long exposures or bulb mode rather than trying to time single strikes. After a test frame, check whether something in the scene is sharp so you can confirm focus hasn’t shifted.