So in fact hardly visible. From 5 mm the image contrast drops rapidly and from 8mm vey fine detail ist just noticable and fine detail is visible with low contrast. From 11mm astigmatism begins to reduce the visibility of fine detail. Subject outlines are rendered softly and in the corners even these details are unsharp. Stopping down naturally improves the quality.
Contrast however stays at a low to meduim level.
There is slight decentring. Modern performance is not possible even at optimum aperture, however. At full aperture distortion is negligable, light fall off is reduced and curvature of field well corrected. At full aperture the center image within a radius of 8mm gives a crisp rendition of subject outlines and fine details.,the corners being very soft. Very fine details are rendered with medium contrast from center to 6mm and with lowish contrast from 6mm to the corners.