Where you cannot use a flash, FAST glass. This allows you to shoot at a lower ISO level, and have better image quality than raising the ISO. Example. 1/60 sec, f/2.8, ISO 1600 1/60 sec, f/5.6, ISO 6400 The 2nd has a 2 stop penalty. You can also raise the shutter speed to stop camera/subject motion better. 1/125 sec, f/2, ISO 1600 When I shoot basket ball with my 18-140, I am shooting at 1/500 sec, f/5.6, ISO=12800. A FAST lens lets me shoot at 1/500 sec, f/2.8, ISO = 3200 or even 1/500 sec, f/2, ISO=1600, for better image quality.