The verb becomes focusing and focused in American and British English (see doubling final consonants).
The noun may take either foci (pronounced foh-see) or focuses as plurals. Although foci is etymologically stronger and far more common, focuses may sound less pretentious to some ears. But in many contexts, focus and its plurals may be best replaced with a stronger and more specific word: Prefer The novel had many themes to The novel had many foci.