Here we measure the smallest change in a face that can be discriminated. A morphing algorithm mixed two faces in variable proportions to create a series of synthetic faces that each differed by a tiny amount. By selecting from this series, a test face could be chosen so as to reach a just noticeable difference from a sample face. Face-discrimination thresholds were about 7% of the average difference between two faces, as quantified by coefficients of a principal components decomposition. This threshold remained constant as the duration of the test face was reduced from 1,000 to 100 msec, and rose quickly for shorter stimulus durations. The behavioral evidence presented here indicates that complex visual processing can be completed within the first 100 msec of the signal, suggesting involvement of feedforward neural mechanisms, and placing constraints on possible computational algorithms employed within the ventral visual pathways.