Sigma has an answer for the question you haven’t yet asked: what to do with your magnetic lens cap once you’ve taken it off the lens? Anticipating confused, non-ferrous photographers, Sigma developed a carbine-attached badge of sorts that can hang from a bag or belt loop. It is in essence, a thin metal doughnut attached by a loop, akin to detective badges seen in any 1980s police television show.
Of special note is the size of the donut. The current lens caps aren’t overly large, coming from Sigma’s C range of lenses, which have relatively small maximum apertures. But the metal doughnut appears to be roughly the same size as the lens caps for this set of lenses. This suggests that Sigma does not intent to provide magnetic caps to its larger lenses in the future, or perhaps it plans on coming out with a larger version of the dongle.
Were it to provide a dongle for the 105 f/1.4 Art, it might be less of a doughnut, and more of a dinner plate. Oh yeah, the dongle and the related lenses are to be announced on the 18th of December, according to Nikishita.