A person or device that focuses; something that concentrates attention or directs optical elements toward a focal point.
From 'focus' + '-er' (agent suffix marking 'one who does'). The '-er' suffix is one of English's most productive ways of creating nouns from verbs.
A microscope's 'focuser' (the knob you turn) uses a precisely-engineered screw mechanism where tiny rotations create tiny vertical movements—engineering so precise it can bring sub-cellular objects into view.
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