I have the cannon 70-200 2.8 and I am about to buy the cannon 23-70 2.8 L. I use a cannon 50d. Light is key, right? So why not keep your Fstop at 2.8 and lower your ISO to 100 and increase your shutter speed to compensate? Unless your shooting the sun, there should be no reason to have anything greater than 2.8, right. From my understanding, the lower the fstop the greater depth of field you have. IE background is blurry, right? so why would you want to use a higher fstop, say f16? Granted, the ONLY time I have used higher Fstops is when I have been playing with "painting with light". so why?