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The newly released Sigma 14mm f1.8 Art lens is discussed in several threads, but it might be convenient to gather info in one dedicated thread. The lens is available in Nikon and Canon mounts. As a guidance to new/potential buyers, here are some findings after playing with three copies of the lens, all in Nikon mounts. The testing has been tailored for night/star/northern light/etc. photography, i.e., with the deep sky as a major component of the image. In the following I only discuss sharpness across the image when the lenses are set wide open (f1.8) and at near infinity. Other optical qualities are, of course, of importance as well (aberration, astigmatism, coma, etc.), as well as other shooting conditions. As mentioned, three copies of the Sigma lens have been tested, all in Nikon mount, but I guess the results are transferable to Canon mounts as well. The tests are done with Nikon D5 and Leica SL, the latter with two Novoflex Nikon-to-Leica adapters (one manual and one electronic): (0) Yes, even infinity needs accurate focusing! Therefore, forget the infinity marking on the lens (as on most lenses). (1) Set the lens to manual focus; it appears that the focusing is less reliable when the lens is set to auto focus (despite auto focus is not activated by any camera buttons). Don't ask why and I can not prove it, this is a suspicion I have based on the testing. (2) Live view on Nikon D5, at full zoom, is only marginally able to guide correct infinity focus. Using a loupe on the back screen is recommended, possibly needed. (3) Live view on Leica SL, at 10x magnification, is much more accurate than the Nikon live view (nothing new here) and can be trusted regarding infinity focus. Thumbs up for the SL! (4) In my case, the manual Novoflex adapter gives even focus throughout most of the image. This is not the case for the electronic adapter; when the center of the image is in focus, the edges smear, and vice versa. So there are differences in the mount precision. This does not imply/prove that manual Novoflex adapters are more accurate than the electronic one, rather that differences between adapters can be expected. So for accurate infinity focus with non-SL lenses, testing of several adapters might be required (actually, I have two manual Novoflex adapters, with one being a `keeper', the other less so). So adapters can be tricky. (5) Irrespective of the FF body used (Nikon D5 or Leica SL), copies of the Sigma 14mm f1.8 ART at f1.8 show differences regarding edge/corner sharpness when the central part of the image is in focus. Vice versa, when the focus point is set near(er) to the corners, the central part of the image may turn soft. So some copies of the lens do not stand the task. Actually, the excellent SL EVF can be used to judge the lens sharpness across most of the image. Set the focus somewhere in the frame, and 'walk' around in the EVF to check the focus throughout the frame (note that we are talking about focus near infinity). (6) Of the three lenses tested on D5 and SL, the latter with the manual Novoflex adapter, one copy is fine - being sharp across the image with only weak softness in the outmost corners. The lens is sharper than the Nikon 14-25 f2.8G zoom, a highly regarded lens (despite the Sigma is shot at f1.8). One copy is perhaps acceptable, the third copy fails wrt edge/corner sharpness. (7) For all three copies of the lens, optimal sharpness across the image appears to be obtained when the focus is put about 1/3 off the centre of the image. (8) I find accurate, manual focussing somewhat tricky with the lens; very small rotation of the focus ring moves the focus point too far/quickly, so I would prefer a longer focus throw length (but I guess this might influence the lens' autofocus). In conclusion, the new Sigma can be fine, even used wide open at f1.8, but testing is needed... Used on the SL, testing of adapters are also needed... Images will follow after some shooting.