When two shapes that differ in orientation or size have to be compared or objects have to be recognized from different viewpoints, the response time and error rate are systematically affected by the size of the geometric difference. In this, report, we argue that these effects are riot necessarily solid evidence for the use of mental transformations and against the use of invariants by the visual system. We report an experiment in which observers were asked to give affine-invariant coordinates of a point located in an affine fame defined by three other points. The angle subtended by the coordinate axes and the ratio of the lengths of their unit vectors systematically affected the measurement errors. This finding demonstrates that the visual system's measurement of invariants need not itself be invariant.