When photographers talk of sweet spots they can mean two things
1. The sweet spot (best focus) on each of your lenses
2. A very small spot that is in focus in an image, while the rest is blurred.
1. To determine the sweet spot for your lenses take several photos at various apertures of a newspaper from about 5 foot. Enlarge them to 100% and you should see a difference and the best focused.
2. My attempt at getting just a small area in focus.