From Photography Tip Blog:

If you want the sharpest pictures that your lens can give you, remember not to use its largest and smallest apertures. Most lenses are optimized to be used in the f/5.6 – f/11 aperture range and give their best performance when stopped down a couple of f-stops from the widest aperture or opened up from the smallest aperture.

The problem with using small aperture sizes is that light waves are affected due to diffraction and though you have great depth of field, you lose out on sharpness.

Large apertures are great for low-light, but unless you have an exceptional, lens its just too difficult to produce lenses that are razor sharp at it’s widest aperture.