puffin21 I have been shooting Sigma 150mm f2.8 Macro lens since 2009 and love the lens…But I rarely shoot with auto focus on any macro lens because it is easier for me to set the focus and then move closer or back away from the subject until in focus and snap the image.
Auto focus when shooting macro in the field is less “profitable” in terms of time and capturing “keeper” images in my experience…Your mileage may vary until you learn the manual focus “rocking” technique.
Check out some of the better field macro photographers around the world, most of them use manual focus and the “rocking” back and forward technique to gain focus.
This method is used by probably 85% - 90% of field insect/bug macro photographers.