Here’s a useful resource I found a while back that many of you might find helpful. SLRGear.com is a website that conducts comprehensive tests on camera lenses, and publishes them in the form of diagrams and illustrations.
One of the features my friends and I have found most useful is the blur index illustration that it provides. This interactive chart helps you find the “sweet spot” for your lens, showing you where the lens is sharpest as you choose a specific focal length and aperture.
From the screenshot above of the Canon 24-70mm blur index chart, you can see that there is a sizable “sweet spot” of sharpness in the center of the frame at 35mm f/2.8. As you move towards the outer edges of the frame, there is less sharpness and more blur. Most of the time you will find that lenses have the largest sweet spot at f/4.0 to f/5.6. If you increase the f-number beyond that, you start losing sharpness again.