I thought that you were using a thin extension tube between your zoom lens and your
camera body. An active tube has a metal conduit running through it so that the auto-
focus of your lens remains electrically coupled to your camera body. The extension tube set
(usually they sell 3 tubes of different thickness) separates your lens from the camera thereby
shifting the lens farther from the sensor. When this shift occurs the lens can focus much
closer to the subject although it can then no longer focus to infinity. Less expensive lenses
like the Sigma will very likely not focus as close as more expensive lenses such as Canon,
Nikon and Olympus. The use of a small thickness extension tube just lets you reduce that
"minimum focus" and get the subject larger in the viewfinder. The closer you focus the more
depth-of-field you need to get everything in focus so you need to use f8, or f11 and that may
need you to crank up the ISO setting to capture enough light. Getting close to your subject,
such as a butterfly, means that you will NOT need to crop as much so using a high ISO setting
will still deliver a reasonably good quality image. I still have a set of extension tubes for my old
film SLR but they do not couple to my current DSLR and they don't have electrical conduits so
even if they would fit there would only be manual focus.
