The longer the lens the harder it is to get acurate manual focus. That's the way it is with me anyway. Take this image above. I was  about 2 meters away and trying to get the focus directly on the tip of the leaf. Did I succeed? Well, yes and no. Part of the tip is in focus and a little out of focus. I thought I'd nailed this one but the slightest movement on your part will render some part on the focus area out of focus. What I'm trying to say is, the depth-of-field is so narrow that if you want to go for these images a tripod is the best way to go.