Sigma 135mm f1.8 Art’s highest magnification is 1:5, which really doesn’t sound like true macro. It should go without saying, that this isn’t enough for portrait shots or even tight shots of butterflies. For shots of butterflies including their surroundings this is enough, however – none of the three butterfly photos were cropped. Furthermore the focal length is long enough, so that you won’t be scaring the skittish insects away while taking their photo.
With an achromatic close-up lens even better results are possible. But looking at it in pragmatic terms, it makes little sense buying a close-up lens for a lens with an 82mm filter thread. Good close-up lenses with that large a filter thread are rare and expensive. Two of the best achromatic lenses on the market, the Raynox DCR-250 and Marumi DHG +5, are only available in sizes up to 67mm and 77mm respectively. The latter costs almost 120 euro. 😉
So it’s clear that the 135mm Art will never replace a real macro lens. But since its strengths lie in a different area, it doesn’t have to. There isn’t a single macro lens that can come close to the 135/1.8 in regard to shallow DOF. 😉