This macro telephoto lens appeared in march 2001 on Sigma website, almost 20 years ago. It is a robust and well built macro lens with a minimum focusing distance of 46cm for a 1:1 ratio. I bought mine used in perfect condition, boxed with the pouch, accessories and paperwork for $250.
I love math, I love macro photography. Combining both of them is at the same time quite fun (to me at least) and useful.
I'm going to talk here about the calculation of magnification for a lens set at closest and infinite focus with any diopter, and the calculation of the resulting maximum and minimum working distances.
I love shooting macro. But not enough to spend thousands of dollars on macro gear.