Plenoptic cameras enable a variety of novel post-processing applications, including refocusing and single-shot 3D imaging. To achieve high accuracy, such applications typically require knowledge of intrinsic camera parameters. One such parameter is the location of the main lens' optical center relative to the sensor, which is required for modeling radially symmetric optical effects. We show that estimating this parameter can be achieved to an accuracy of less than half a pixel by utilising the symmetry inherent in each micro-image. Further, we show that estimating this parameter separately allows all other intrinsic camera parameters to be estimated with higher accuracy than can be achieved using a single optimization scheme, and leads to better vignetting correction than with an inaccurate optical center.