More specifically, it's longitudinal CA (LoCA, as opposed to lateral CA). LoCA occurs only in high contrast areas with fast lenses (not just your Sigma!) shot wide open, occurs across the frame (lateral CA affects the edges more), and is worst in out of focus areas. LoCA is also called 'bokeh fringing' and is much more difficult to correct in post than lateral CA (often, LoCA cannot be corrected).


However, unlike lateral CA, LoCA is eliminated by stopping down the lens. It also helps to minimize strong contrasts (e.g. chrome) when shooting wide open, or if they are in the image, it's best if they are in focus, as shown in a previous post where I did some testing of my 85L for LoCA.


Hope that helps...


--John