Sentence:  Not long ago, it was difficult to produce photographs of tiny creatures with every part in focus. That's because the lenses that are excellent at magnifying tiny subjects produce a narrow depth of field. A photo processing <WSD>technique</WSD> called focus stacking has changed that. 
Meaning of the word marked with <WSD> tags:  The practical aspects of a given art, occupation etc.; formal requirements. (from 19th c.)