The reason(s) for buying a faster lens vs the slower lens, even though it costs more, in response to someone else's comment earlier in the thread:

  • The faster lens is easier to focus in low light
  • The faster lens can let you use a faster shutter speed in low light, enabling you to capture an image you couldn't with the slower lens
  • The faster lens may have a more aesthetically pleasing appearance at or near wide-open than the slower lens does
  • The faster lens will let you blur out backgrounds and foregrounds more than the slower lens does (see item above)

and then there's always, perhaps the most important reason of all-

  • the faster lens proves you're a better photographer because you can afford more expensive equipment, and gives you an excuse to toss around terms like bokeh, circle of confusion, and diffraction limited... (cough cough)