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 bits of multiple <WSD>digital</WSD> images, focus stacking is a boon to biologists seeking full focus on a micron scale. 
Meaning of the word marked with <WSD> tags:  Property of representing values as discrete, usually binary, numbers rather than a continuous spectrum.