This is pretty self explanatory, yet I will try to make it more difficult than it is:
Because focusing determines the "focus" of the image, it is critical to get it right. But when doing so, there can be desired areas that are left out of it.
Here is how you can get around it:
1. Focus on your subject, light the subject
2. Shift the focus to your background/foreground/other subject, and light that too
The hard part is adjusting the lens to where it needs to be. Take note of the distance labels on the focusing ring, and remember them.
Using the same technique, you can make a part of the image completely out of focus. Just spin that focusing ring!