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