Exactly what I was looking for, a light, bright UWW to be able to photograph stars. Otherwise, the wide-angle range is covered several times for me (Nikon 16-35, Nikon 20mm/1.8, Sigma 12-24mm second version). So I ordered it and tried it out, I got hold of one from a Dutch retailer via Amazon in September, even with a 5-year guarantee after registration. My test setup in the basement, a wall unit with a test panel in the middle and one in each corner. I had already tested other lenses with it and discovered weaknesses (slightly muddy corners due to decentering) that I had not noticed in everyday life. In any case, the surprise, the K&F Concept already shows almost the same sharpness in all corners as in the middle at an aperture of 2.8! The further stopping down mainly brings an improvement in vignetting, from f4 this is almost non-existent (there is currently no lens profile for the Nikon version). For this test, the camera (D800) was of course placed on a tripod and aligned with the virtual horizon. The parallelism to the wall was checked with a meter stick and an auxiliary ruler. The so-called glacier fracture test no longer works with the extreme focal length, since the corners warp due to the tilt of the camera when panning (physically caused). Enough theory, the moment of truth came in the Dolomites during one of my first attempts at milky way photography. The conditions were ideal, the result fantastic (first picture, not the final edit yet). The last image shows a 100% crop of the same but unedited RAW image from the top right corner(!), only the exposure was increased by one stop. I would be happy if there would soon be a lens profile from K&F Concept for the Nikon version, unfortunately nobody answered my email. Otherwise I am very pleased with the results. I sent the Irix 15mm back straight away, it was neither visually nor haptically appealing, only the filter suitability would have spoken for it.