Shack-Hartmann wavefront sensors (SHWFS) usually have fixed subaperture areas on the detector so the minimum and maximum amount of wavefront departure, or the dynamic range of measurement, is fixed. We introduce an active approach, named A-SHWFS (Adaptive Shack Hartmann Wavefront Sensor) to reconfigure detection subaperture areas by blocking / unblocking desired lenslets using an electronically modulated mask which increases / decreases the measurable aberration magnitude. This is achieved through an LCD (liquid crystal display) panel placed in front of the lenslet array. Depending on the control signal sent to the LCD, the variable, application-dependent blocking pattern (horizontal, vertical, diagonal, uneven) makes this an adaptive and efficient sensor with a variable dynamic range of measurement. This scheme is also useful for regional blocking, when the wavefront is only severely aberrated in a limited region.