As others have posted, shooting wide open your depth of field is going to be shallow. By stopping down, it will thicken up making focusing and maintaining focus a lot easier. Distance is also at play here, in terms of being close to the camera, your DoF is also going to be thin.
The other aspect to this is, that there are some disadvantages to shooting wide open. Vignetting, CA (purple fringing), and resolution are all in play here. Taking a look at a review for the lens, you are going to want to stop down to about f4 to f5.6, to reduce vignetting and CA, plus your overall image resolution across the entire face of the lens will improve.
If light becomes an issue with stopping down, then some lighting improvements would certainly help with things.