The motion estimation unit (100) comprises a first summation means (106) for calculating match errors of a number of candidate motion vectors of a segment (116) of a first image (118). The motion vector with the lowest match error can be assigned to the segment (116) as the estimated motion vector. The match error is based on summation of absolute differences between pixel values of the segment (116) and pixel values of a second image (120). In order to estimate the deviation of the estimated motion from the true motion vector, i.e. the motion vector error (130), the motion estimation unit (100) further comprises a second summation means (108) for calculating a variance parameter by summation of absolute differences between pixel values of the segment (116) and pixel values of the first image (118) and estimation means (110) for estimating a motion vector error (130) by comparing the match error with the variance parameter.