The theory of operation behind the aperture sight is often stated that the human eye will automatically center the front sight when looking through the rear aperture, thus ensuring accuracy. However, aperture sights are accurate even if the front sight is not centered in the rear aperture due to a phenomenon called parallax suppression. This is because, when the aperture is smaller than the eye's pupil diameter, the aperture itself becomes the entrance pupil for the entire optical system of target, front sight post, rear aperture, and eye. As long as the aperture's diameter is completely contained within the eye's pupil diameter, the exact visual location of the front sight post within the rear aperture ring does not affect the accuracy, and accuracy only starts to degrade slightly due to parallax shift as the aperture's diameter begins to encroach on the outside of the eye's pupil diameter. An additional benefit to aperture sights is that smaller apertures provide greater depth of field, making the target less blurry when focusing on the front sight.