In article
,
RichA wrote:

Heat expansion and contraction, which effects optical performance. So
they've introduced something else to compensate for it. Because
aluminum and magnesium don't suffer nearly as much thermally-related
shape changes as plastic, older lenses and new high-end principally
metal lenses do not have the problem.

Sigma's biggest problem is their lying about the crappy technology used
in their overpriced, mediocre equipment.