Taken with the OLD lens.
Taken with the NEW lens.
I recently bought a new lens for my SLR. It's a fixed focal length 50mm lens, and is much brighter than the zoom that came with it. I wanted to see the improvement, so I asked my son to swing his arm quickly and I shot these two photos. The one where you see his arm is with the new lens. Both were shot in existing light, only moments apart, and with the aperture wide open. It's not scientific (i.e. his arm's speed likely differed), but the difference in the shots is apparent.