The general approach to wavefront sensorless adaptive optics (or “sensorless AO” for short) relies upon the optimisation of a measurement that is known to be related to the aberration content. For example, in an adaptive laser focussing system, one might maximise the intensity at the centre of the focussed beam. In image-based AO systems, which are a sub-category of sensorless AO systems, an appropriately chosen image property is optimised. In certain microscopes (e.g. confocal or multi-photon microscopes) the total image intensity (sum of all pixel values) is an appropriate optimisation metric, as it exhibits a maximum value when no aberration is present. If the aberration in the system is non-zero due to refractive index variations in the specimen, then the value of the metric will be lower than its optimal value. The goal of the sensorless AO routine would be to use the adaptive element to maximise the metric by minimising the total aberration in the system.