As stated a couple of times already Auto Focus (AF) is convenient and some cases a life saver (speed).

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I hate it.

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AF uses contrasty areas to set the focusing point.  It has no regard for what you want.  If there is a contrasty area in front or behind your subject, that's it.  It wil lock onto that.  When I see captures posted in forums that are soft in 95% of the cases this is due to the AF locking close to subject but not on it.  That is unacceptable.  If you think using a greater DOF will correct the softness, think again.  A DOF creates an acceptable field of sharpness, it does not sharpen anything, quite the contrary even.

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When I read folks complaining that their 'prime' lenses are not all that sharp the first question that comes to my mind is:  Are you using AF?  Then I think of the lens aperture and finally of the subject/camera distance*...

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There are several AF modes in your cameras.  Some work better than other but that is camera and manufacturer dependent.  Experiment.

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AF will have difficulties to lock onto clouds, with night time low light scenes...  Basically anything that has no contrast.  The focus 'helper' (flashing light) is of little help and really an annoyance in these situations.

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* Prime lenses are specialized and work best within strict parameters that include aperture and distance.