Why is it desirable that microscope objective be parfocal?

it is desirable for the microscope to be parfocal because then you do not have to refocus the microscope everytime you change objective lenses.

r=rho l/a where a is the area, l is the length and rho is resistivity so the greater the area the lesser the resistance thats why the earth pin is thicker bcos current follow the shortest path, so less resistive .