Variation of stereoscopic acuity with observation distance and eye movement characteristics

It has puzzled research workers in perception that stereoscopic acuity decreases with decreasing observation distance, or, equivalently, increasing angle of convergence. Experimental results presented in this paper, as well as evidence published previously, show that the reduction in acuity might be due to rotation of the eyes around their visual axes during convergence which would introduce a disparity for the test lines. Stereoscopic acuity is known to be worse for disparate images. It was proved experimentally that cyclodisparity did lead to a decrease of stereoscopic acuity, supporting this explanation.