When opening and closing the iris, you can quickly see how the sharpness of the target image changes. In order to achieve an optimal target image, it is necessary to try out a little at the beginning:
In the range 1.0 to 2.0, the best results are achieved with the rear sights-iris disc . The eye looks uncocked through the iris diaphragms at the mirror. The plane of focus lies somewhere between aperture insert and the mirror. So you don't see either the disk mirror or aperture insert particularly sharply. The rear sights-iris disc is varied until the aperture insert appears sharp, with the mirror slightly out of focus.