Since lens manufacturers invented the lens aperture, photographers have been talking about the "sweet spot." What is it?
The sweet spot is the sharpest point of your lens. If you want to create photographs that take full advantage of the sharpness of your lens you need to choose an f/stop that will utilize the sweet spot. To understand the sweet spot you need to understand how lenses are made. In a nutshell, when you grind glass (in the traditional manner) the cleanest and clearest part of the lens is the middle part. Around the edges we often find aberrations which tend to refract light poorly and therefore render the areas on the edges of your image blurry. The unsharp area is often termed as fall off. Therefore if you only use the middle portion of the lens you will not see any blurriness across the entirety of the photograph. So this area is deemed the sweet spot.
The question then is how do you eliminate the outer edge of the glass where the aberrations exist? When you take a picture using the widest aperture available on your lens, you are using the entirety of your glass including the area on the edges.To cut out the fall off, you would want to close your lens down at least one stop from the maximum. In other words if you have a f/2.0 lens you would want to shoot at f/2.8 to eliminate the fall off. However, this does not mean that you are shooting in the sweet spot as yet. To shoot in the sweet spot, you would want to use the middle of the glass. To do this you would use the middle f-stop of your lens. In other words, if your lens had a range of f/2.8 to f/22 then you would want to shoot at f/8. If your lens was a f/1.8 to f/22 then you would want to shoot at f/5.6. As you close down, you are utilizing more and more of the middle portion of the glass and therefore the sharpest area of glass - the sweet spot.
The information above is what we have been told for a century and will continue to hold true if we continue to manufacture lenses in the traditional way. Luckily thanks to the advancement of glass production and computer simulation in recent years, aberrations on our lenses is going to be a thing of the past. Manufacturers are changing the way in which they produce glass and as a result of this we are now beginning to see lenses without any fall off at all. The manufacturing process is allowing production of lenses without aberrations. Through the use of computer simulation, manufacturers are also redesigning the shape of the each lens within the barrel to also eliminate fall off. Furthermore, manufacturers are also experimenting with the way sensors are made so that they better utilize the glass. For instance, some are curving the sensor in an effort to create a much sharper image all the way across the frame with no fall off.
With the combination of new glass and sensor manufacturing, it is clear that in the near future we will no longer need to consider the sweet spot because the whole frame will be sweet!