When light travels though the lens, the aperture blades can reduce the diameter of the hole the light travels through. Most lenses can close down the aperture to a very, very small area – the size of a pinhead or even smaller.
When light has to travel through such a small hole it simply changes slightly. This can get very technical, but the effect is that your images appear soft, i.e. as if out of focus – even when you have focused perfectly.
Photographers debate exactly when the effect sets in and how large the effect is. Macro photographers often shoot at crazy high f-stop numbers (closed down apertures) and seem to be doing fine, and in many find that the effect of diffraction is exaggerated.
The best you can do is to notice if your lens/camera combination gives diffraction – I think you should start to worry about this from around F/16 and onwards.
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