By solving the two-dimensional Poisson equation for fibers with circular internal electrodes, we show that the parallel-plate approximation incurs significant error in the determination of the electric field applied to the fiber core. This also affects the value of
measured in electro-optical fibers. The exact solution of Poisson’s equation is used to solve the field applied to the core of a photonic crystal fiber, where the many holes screen the field. A universal curve is derived to allow using the simplifying parallel plate approximation and correcting for the error in symmetric fibers with round holes.