The logic for AF-C having a default setting of Release is that it is usually better to get a slightly out of focus image of a moving subject than no image at all. AF-S with a default of Focus assumes you are taking a single frame and will take whatever time is required to get a sharp image. I always use AF-C with it set on release. Images are generally sharp. I've had too many missed frames caused by Focus priority. Now the situation is different if you are trying to sort through focus or sharpness problems. In that case you might set AF-C to Focus priority. I still think that is a bit backwards - because most situaitons that need Focus priority will be handled through AF-S. If you are testing and want to make sure you are in focus, use AF-S with the default setting of Focus priority. There are reasons for using AF-C with Focus priority, but they are for uncommon situations. Another area of caution for AF-C is the focus and recompose technique. AF-C uses Focus tracking, and the camera may confuse Focus and Recompose with a moving subject that needs to be tracked. Don't use Focus and Recompose if you are having sharpness issues - it just adds another variable.