Sentence:  Not long ago, it was difficult to produce photographs of tiny creatures with every part in focus....A photo processing technique called focus stacking has changed that. Developed as a tool to electronically combine the sharpest <WSD>bits</WSD> of multiple digital images, focus stacking is a boon to biologists seeking full focus on a micron scale. 
Meaning of the word marked with <WSD> tags:  A portion of something.