The “Thermally Stable Composite” technology is pretty much never talked about. As a part of the recent bump in build quality from Sigma, I think it’s worth a few words.
All Global vision lenses benefit from it, as well as a few older lenses like the 18-250mm f/3.5-6.3 DC OS.
Sigma is rather coy regarding their TSC. From what we can gather here and there, it consists of a compound material that is said to have “high affinity to metal parts”, to “consistently performs well at extreme temperatures”, and to “reduce the size and weight of the lens” in which it is used. Let’s clarify a bit.
While regular polycarbonate is more prone to thermal contraction than aluminum, TSC is meant to have similar shrinkage properties to this metal, in order to prevent some parts of the lens from slightly contracting, bending or stretching when they are subject to extreme temperatures and temperature variations. Extreme does not necessarily mean Saharan heat or polar cold – 75°F does not have the same effect as 35°F on a material, and sometimes these values are met within minutes. Think inside-outside, and vice versa. It’s a matter of a few microns, but when it comes to optical devices, precision and stability are important.
May the mechanical quality and stability of a lens subtly degrade over the years due to the various environmental changes it inevitably undergoes? Probably. We don’t know about how Sigma’s TSC-equipped lenses will fare over the years, but the advent of such a material can only be welcome.