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Diffraction
Diffraction is an optical effect which can limit the total resolution of your photography-- no matter how many megapixels your camera may have. Ordinarily light travels in straight lines through uniform air, however it begins to disperse or "diffract" when squeezed through a small hole (such as your camera's aperture). This effect is normally negligible, but increases for very small apertures. Since photographers pursuing better sharpness use smaller apertures to achieve a greater depth of field, at some aperture the softening effects of diffraction offset any gain in sharpness due to better depth of field. When this occurs your camera optics are said to have become diffraction limited. Knowing this limit can help you to avoid any subsequent softening, and the unnecessarily long exposure time or high ISO speed required for such a small aperture.
ALPAs rule of thumb: Digital large format lenses are optimized for use at full aperture. Nevertheless the photographic result will also benefit from stopping down. These two elements intersect normally between f8 and f11. Here you will often the optimal result. But hey, sometimes it is better to get a picture then missing the opportunity. Therefore try to use the optimal f-stop but if there is too much or too little light just use the best compromise.
For a practical illustration of the effect of distortion in relation to the aperture have a look at the link below.
