The zone of sharpness is called the depth-of-field, it extends in front of and behind the focus point.
Three main factors to control the depth-of-field:
1 The aperture.
* For maximum sharpness, use small apertures - f/16, or f/22.
The further you stop down, the softer it gets due to light Diffraction and will eventually lose sharpness.
* If you want to concentrate attention on just one part of the scene, and throw the rest out-of-focus, you should select a large aperture typically be f/3.5 or f/4.5.
2 The focal length of the lens.
* A wide-angle lens gives extensive depth-of-field, which makes it easy to keep everything in focus.
The wider the angle-of-view, the greater the depth-of-field.
* A telephoto lens gives a limited depth-of-field. The longer the focal length, the more restricted the zone of sharpness is.
3 The Camera-to-Subject Distance
* The closer you get to the subject the more limited the depth. When shooting close-up subjects it can extend to just a few millimetres in front of and behind the subject.
* As a rule-of-thumb, there's twice more depth-of-field behind the subject than in front of it.
You can guess the hyper focal focusing distance, but life is much easier if your lens is marked with a depth-of-field scale. If it does not have a scale, line up the infinity mark against the mark for the aperture you've set and, although the image in the viewfinder will look out-of-focus, the finished image will be sharp from front to back.
The picture shows a lens set to f/22.
Everything from infinity to 2m is sharp.