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 <WSD>narrow</WSD> depth of field. A photo processing technique called focus stacking has changed that. 
Meaning of the word marked with <WSD> tags:  Having a small width; not wide; having opposite edges or sides that are close, especially by comparison to length or depth.