Micro-electrical-mechanical-systems (MEMS) deformable mirrors (DMs) are under study at the Laboratory for Adaptive Optics for inclusion in possible future adaptive optics systems, including open loop or extreme adaptive optics (ExAO) systems. MEMS DMs have several advantages in these areas because of low (to zero) hysterisis and high actuator counts. In this paper, we present work in the area of high-contrast adaptive optics systems, such as those needed to image extrasolar planets. These are known to require excellent wavefront control and diffraction suppression. On the ExAO testbed we have already demonstrated wavefront control of better than 1 nm rms within controllable spatial frequencies, however, corresponding contrast measurements are limited by amplitude variations, including variations introduced by the MEMS. Results from experimental measurements and wave optic simulations on the ExAO testbed will be presented. In particular the effect of small scale MEMS structures on amplitude variations and ultimately high-contrast far field measurements will be examined. Experimental results include interferometer measurements of phase and amplitude using the phase shifting diffraction interferometer, direct imaging of the pupil, and far-field imaging.© (2008) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.