I do love the lens, but i still struggle to get real sharp and correct focus photos. I think 95% of my photos can go straight into the bin. Biggest problem is not the lens or me but the wind. I also don't use a flash when i go out for macro shots and shoot only handheld. Usually i go for f/8 or f/11. I also use it sometimes on my sony a6000, works fine on that one too even without image stabilisation.
Yes, same for me, it is hard to nail the focus most of the time. The wind, along with the movement of the insects makes things very frustrating at times. I just don't shoot if there's more than a light breeze, unless I can find a calm spot or strong stems that don't move much. Dandelions are good for that situation.
I use a flash because the lighting is more consistent and predictable. I find that natural light changes too often and is sometimes too harsh and uncontrollable. I only shoot handheld as well.
F/8 is usually my home, but I do use f/2.8 to f/11 depending on my tolerance for missed shots on any given day, lol. I find that if I want extremely sharp detail I have to go somewhere between f/2.8 and f/8. Anything above f/8 starts to soften, but I can get more of the subject into focus and miss less shots..
