Currently, polarization calibration for full Stokes imaging polarimeters is limited by the apertures of the retarders. In this paper, an improved polarization calibration with large apertures in full field of view for full Stokes imaging polarimeters based on liquid-crystal variable retarders is proposed and investigated theoretically and experimentally. The experimental precision of polarization calibration is 1.7% for linear polarization states and 8.8% for circular ones for an imaging polarimeter with a 100 mm aperture and 10° field of view. The feasibility for full Stokes polarization image is also confirmed in experiment for identifying objects due to degree of polarization and degree of circular polarization images.