The purpose of the work is to specify tolerances for the alignment of binocular instruments. Contrast threshold levels, time-to-fuse and accommodation were recorded for binocular view of a target at a distance of 1·3 m, in a group of six observers and under prismatic conditions requiring a range of different horizontal and vertical fusional responses. It is confirmed that the derived values of binocular summation are maximal when the prism arrangement corresponds to natural viewing of the target. Values of convergence, divergence or supravergence in excess of 1° from this natural condition produce significant reductions in binocular summation. Measurements of time-to-fuse reveal rate differences between the horizontal and vertical fusional mechanisms and further produce a more stringent condition, 0·5°, on supravergence tolerance. Measurements using a speckle optometer show only small changes in accommodation over the whole investigated ranges of vergence and provide clues to the mechanism of the observed reduction in binocular summation.