This paper presents the design of a flexure based focusing system, part of the Close-Up Imager (CLUPI) instrument which is planned to be integrated on the future ExoMars NASA-ESA Mars Rover to be launched in 2018. Various design trade-offs are discussed, in particular the selection between a non-symmetric isostatic kinematic flexure arrangement and a symmetric but over-constrained arrangement. The selected solution is based on an original architecture consisting of three identical EDMmachined compliant plates connecting the mobile lens assembly to the fixed outer frame. This novel technical solution has potential use for all kinds of focusing mechanisms dedicated to harsh environments.