Diffraction and depth of field
Diffraction occurs when you shoot at high f-numbers and makes your images appear less sharp. It is caused by the way the light travels through the aperture diaphragm before it reaches the sensor. When you shoot at very narrow apertures, such as f/22, the wave pattern of the light hitting the sensor spreads out and, instead of covering just one pixel on the sensor, falls across many pixels, making the whole image look softer. So even though your depth of field is increased at apertures like f/22, the overall image becomes less sharp. It’s more noticeable on higher-resolution sensors, and can be an issue on cameras with smaller DX sensors, where you should avoid apertures higher than f/11 or f/14.