If I use the centre focus point on the D800, it will focus accurately on a LensAlign ruler, about a foot away. If I use either far right or left focus points, I get at least an inch of backfocus. I don't get this with a 50mm and it's consistent across three D800 bodies.
If anyone else can try this with a D800 and 15mm Sigma I would be grateful - any old ruler should do. Perhaps there's some optical property that causes this, but it's a nuisance. Or is my lens faulty? Live view will focus accurately.