Most implementations of adaptive optics in microscopes have not employed a wavefront sensor, but have instead
used sensorless aberration correction methods. In these systems, the aberration is determined indirectly through
the optimisation of a quality metric, such as image intensity. We explain the principles behind this approach and
show how efficient correction schemes can be developed using suitable mathematical models. In particular, we
explain how the correct choice of aberration expansion enables the independent measurement of each aberration
mode via a simple quadratic maximisation.