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This paper presents an H∞ controller designed to cancel dynamic wavefront aberrations in an adaptive optics (AO) system based on a magnetic-fluid-deformable mirror (MFDM). MFDMs are a recently proposed novel type of active optical elements called wavefront correctors, which constitute the central part of AO systems. They offer cost and performance advantages over existing wavefront correctors. MFDMs have been found particularly suitable for ophthalmic imaging systems where they can be used to compensate for the complex optical aberrations in the eye that blur the images of the internal parts of the eye. However, their practical implementation in clinical devices is hampered by the lack of effective methods to control the shape of their deformable surface. Specifically, control algorithms that can be used to cancel dynamically varying wavefront aberrations need to be developed. This paper presents one such control algorithm that can be used to compensate for high-order time-varying optical aberrations using an MFDM. The control algorithm is developed using the mixed-sensitivity H∞ design method that enables the tracking of the desired MFDM surface shape and also limits the magnitude of the control currents applied to the MFDM. Experimental results showing the performance of a closed-loop system comprising the developed controller and a 19-channel prototype MFDM are presented.