izoštriti

1. to sharpen
   Moram da izoštrim olovku. — I need to sharpen my pencil.
2. (of an image) to bring into focus
   Izoštri sliku pre nego što slikaš. — Bring it into focus before you take the picture.
3. (of senses, skills, views) to sharpen; to hone
   Rad u kuhinji mu je izoštrio čulo ukusa. — Working in a kitchen sharpened his sense of taste.

The imperfective is izoštravati; for a knife naoštriti is more common.