Not using the optimum aperture
Although there will be situations when you want to use a large aperture to help you separate a sharp subject from a blurred background, there will be other times when you want more of a scene to appear sharply focused. It might be tempting to reach for the smallest aperture on the lens, but this actually leads to softer pictures due to the effects of diffraction – essentially incoming light rays being bent out of shape by the aperture blades, which is more noticeable at small apertures.
It’s often preferable to sacrifice some depth of field in order to deliver an image where details are pin-sharp. This is often in the middle of a lens’s aperture range – typically around f/8 to f/11, although this varies from lens to lens.
An aperture of f/22 may not give bitingly sharp results thanks to the effects of diffraction, whereas an aperture of f/5.6 may not offer enough depth of field for a scenic shot