conversions for a particular display or light valve the system 24 may implement the differential focus corrections automatically in response to tilt information from the tilt indicator 23 or using the feedback technique illustrated in fig1 and 4 alternatively the power of the optical element 21 may be fine tuned by hand by the operator as still another alternative the focus may be adjusted by simply measuring the angle of tilt and manually inputting the appropriate correction to the system 24 and element 21 normally projectors are designed to project upwardly or downwardly for example table mounted projectors usually project upwardly to the display surface conversely ceiling mounted projectors are designed to project downwardly to the display screen thus the optical element 21 may correct for the keystoning effect arising from either upward or downward projection that is the optical element 21 need only correct for either the keystone with a wider bottom edge or wider top edge this simplifies the design of the optical element of course more than one optical element may be provided to enable the system to work on either upwardly or downwardly projected images for example a shutter mechanism may be provided to move one or the other of two electrically variable optical elements into the projection axis depending on whether the projector projects upwardly or downwardly in addition a pair of such elements may be provided in series.