Before using a new lens, we pay attention to its sharpness and how to achieve the best result. A common misconception is achieving that with the smallest aperture the lens can reach, which is wrong. Each lens has a 'sweet spot' where sharpness is the best. In the following example, we have 3 shots made with a Canon EF 135mm f/2L, cropped at 100%. If you notice, f/8 is sharper than f/16 and f/32.
At a smaller aperture, the light refraction over the blades of the iris diaphragm is the cause of the lack of sharpness. So, it's important to test your new lens for the best aperture for it.. Remember: no two lenses are alike.