Always test my lenses for decentering wide open by taking one horizontal normal shot and then the same shot with the camera upside down. Always worked to show up any decentering, but not so with the Sigma 30mm (I got the older version, just 109 euros). When I take test shots with this one, the normal shots turn out just fine, very sharp across the frame wide open. The upside downs shot consistently exhibit decentering on the left side, i.e. the right side should show the same decentering in normal shots, but it does not. (Vertical shots in both directions all look OK to my eyes.) So basically, for my pedantic but real-life photography purposes this particular copy is good. But what is happening to the lens when taking upside down photos is quite interesting. Definitely not down to my hands/holding technique because no other lens I have tested over the years exhibitis such inconsistency - they are either equally sharp or equally decentered in both normal and upside down shots. My hands are steady (can't use the tripod for placing the camera upside down). I have also been able to reproduce this inconsistency on two camera bodies. Could it have something to do with the Sigma floating focus mechanism (remember how it is prone to rattling when switched off)? Also what is more realistic to assume, that Sigma actually optimised that mechanism for normal (not upside down) shooting or it is sample variation and there are copies of this lens out there that are great when shooting upside down but poor when shooting normally? And most importantly, do I need to get a life?