1. A method of projecting a pattern from a patterning device onto a substrate using a projection system, the method comprising using an optical phase adjustment apparatus in the projection system to apply a phase modification to radiation which has been diffracted from an assist feature of the pattern, the phase modification acting to reduce the size of an assist feature image exposed in resist on the substrate or prevent printing of the assist feature image in the resist on the substrate, while maintaining a contribution of the assist feature image to an image enhancement of a functional feature of the pattern.