Thanks!
I change focus manually. Cable release in left hand, focus ring in right, shoot, turn a tiny fraction and repeat, carefully watching the zebra stripes as they track their way down the insect. It's suboptimal, but I can't justify buying the A7Rv, which does have focus bracketing, merely for that. Maybe I'll get an A7Rvi, when and if it's announced.
Tony (my teacher) has just acquired an Olympus which has bracketing. Says it's changed his life.
Jeremy