focus vs focus on

focus is a noun but focus on is not a noun.

Word	Noun	Adjective	Verb	Adverb
focus	Yes	No	Yes	No
focus on	No	No	Yes	No

focus (noun)	focus on (noun)
maximum clarity or distinctness of an image rendered by an optical system
maximum clarity or distinctness of an idea
the concentration of attention or energy on something
a fixed reference point on the concave side of a conic section
a point of convergence of light (or other radiation) or a point from which it diverges
a central point or locus of an infection in an organism
special emphasis attached to something

focus (verb)	focus on (verb)
become focussed or come into focus	center upon
put (an image) into focus
direct one's attention on something
bring into focus or alignment; to converge or cause to converge; of ideas or emotions
cause to converge on or toward a central point