Depth-of-field is a valuable composition tool.
When you focus on your subject—that's where the focus is the sharpest.
However—there's a zone in front and behind the subject that's acceptably sharp.
This acceptably sharp zone is the depth-of-field.
Compare the two photographs below.
In each, the camera was focused on the crystal paper weight.
The top photograph has very little in focus.
The depth-of-field is shallow.
There's more in focus in the bottom photograph.
It has more depth-of-field.
The amount in focus, the depth-of-field, depends on three factors.
1) Lens opening
2) Distance from the subject
3) Sensor size
We'll look at each of these factors.