The ability to judge the separation between two target lines deteriorates as the base separation increases. Several lines of evidence suggest that this may be due to larger base separations being processed by mechanisms that cover larger areas and which have a greater associated positional uncertainty as a consequence. Separation discrimination was measured as a function of base separation with randomly jittering targets. As predicted from the above models the resistance to positional noise increased in proportion to base separation of the targets. The data is incompatible with the suggestion that resistance to positional noise declines when the extent of the noise exceeds fixational instability.