sentences:  What is considered "fast" has evolved to lower f-numbers over the years, due to advances in lens design, optical manufacturing, quality of glass, optical coatings, and the move toward smaller imaging formats.For example, the 1911 Encyclopædia Britannica states that "...[Lenses] are also sometimes classified according to their rapidity, as expressed by their effective apertures, into extra rapid, with apertures larger than 6; rapid, with apertures from 6 to 8; slow, with apertures less than 11.