Hi Dave, the "Sweet Spot" refers the aperture at which a lens works best. The aperture at which it produces the sharpest image. It may come as a surprise to you but lenses are not consistently sharp across all apertures. This is due to the physical make up of them and the physics involved in their manufacture. The sweet spot may vary from lens to lens purely because of the manufacturing involved. This is one reason why top glass is valued not only for its optical properties but also the consistent quality control of the manufacturer. With many lenses f8-f11 is the aperture at which one expects the lens to perform best, hence that is it's sweet spot, but not all lenses will conform to this. With a 50 mm f1.4 Nikkor many say the sweet spot is around f5.6 - f8. With a 70-300 mm zoom it may have different sweet spots depending upon the combination of focal length ie: it may have a different sweet spot at 70 mm than at 300 mm.
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