Sentence:  The <WSD>single</WSD>-imaging optic of the mammalian eye offers some distinct visual advantages. Such lenses can take in photons from a wide range of angles, increasing light sensitivity. They also have high spatial resolution, resolving incoming images in minute detail. It's therefore not surprising that most cameras mimic this arrangement. 
Meaning of the word marked with <WSD> tags:  Not accompanied by anything else; one in number.