When photographers first learn about depth-of-field, they often become small-lens-opening enthusiasts.
They use the smallest possible lens opening—all of the time.
Their photographs may, unexpectedly, not be sharp.
While small lens openings have more depth-of-field, they also have more diffraction.
Diffraction degrades the sharpness of the photograph.
Let's say you're photographing an apple.
Light reflects off of the apple, goes through the lens opening, and strikes the sensor.
However, some of the light waves also hit the edge of the lens opening.
These light waves reflect off in new directions.
The light waves coming directly from the apple are degraded by the waves that are coming from the edge of the lens opening.
The two sets of waves interact with each other, reinforcing and canceling each other.
This interaction degrades sharpness.
At large lens openings, the amount of the diffracted light is a small portion of the total amount of light reaching the sensor.
At small lens openings, the percentage of diffracted light increases.
For photographs that will be enlarged greatly, avoid using lens openings smaller than the focal length of the lens divided by four.
For example, let's say you're using a 50mm focal length.
50mm / 4 = 12.5
The closest lens opening to 12.5 is f/11.
Do not go smaller than f/11 when using a 50mm focal length, for optimum results.
Point-and-shoot cameras often have f/8 as the smallest lens opening.
At a focal length of 6mm, the physical size of the lens opening at f/8 is .75mm.
.75mm is a tiny lens opening.
If the camera had smaller lens openings, such as f/11 and f/16, diffraction would become evident at these smaller lens openings.