Insisting that the focus locks on, by repeated exercise is ok... a/ if you are within range to pull the eye into focus in the first place. b/ young enough.
Sadly, as the eye ages, the myriad suspension points that apply force on the lense, grow further in towards the centre of the pupil. Their angular force becomes diminished and the eye cannot be made to accommodate over the same range.
Excellent micro-photography in Scientific American about (guessing) 15 years ago.