We all know that reducing the aperture reduces the size of the circle of confusion created by the lens which gives the effect of increased depth of field. What many photographers do not realize is that there is a point after which the small aperture begins to diffract or divert he light rays creating a larger rather than a smaller point of light. This point, known as the airy disc, rarely had an impact on conventional film photography.
In digital capture, the size and position of the pixel elements are fixed and if the airy disc becomes larger than the size of the pixel the diffraction begins to degrade the effective resolution. This is a simplified explanation, of course, and the diffraction effect varies with the size of the sensor and the number of pixels on the sensor. The smaller the pixel “sites” the faster this becomes an issue.
In a typical camera diffraction limitation will occur more quickly on a higher resolution sensor as the pixels sites are smaller. Small sensor cameras like point and shoot cameras suffer much more quickly from diffraction. This is why you see cameras with small sensors limit the smallest apertures to numbers like f/5.6 or even less. Since smaller sensors have effectively greater depth of field at larger apertures, this is not a problem in normal photography.
I suggest you visit Cambridge in Colour’s website for a thorough explanation of diffraction and a calculator that will help you determine the optimum apertures for your camera.
Also check out their page on depth of field calculation to see just how small your aperture needs to be on your camera to effectively render sharp focus. You may be surprised at the results.
Images made at typical landscape distances rarely require apertures smaller than f/8 or f/11 for effective sharp focus over the entire field. That means that apertures like f/16 or f/22 are not helping you make sharper images, they are actually creating diffraction that is making your images less sharp. Add to that the longer exposures needed you are adding more elements to your capture that can degrade rather than enhance your photography.