How to find the sweet spot of a lens?

I hear about the sweet spots on lenses, but I don't really know what that means… Is the image quality best/sharpest? Is the "spot" a certain focal length?

I have two zoom Nikon kit lenses, a standard 18-55mm f/3.5-5.6, and a 55-200mm f/4.0-5.6… Does anyone else have experience with these lenses? Can someone please fill me in, and if you've used or are familiar with these lenses, please give me some idea about mine? Any help would be much appreciated, Thanks!

There's no "sweet spot."

I've never heard that.

With wide lenses with wide apertures, you will get blur toward the edges. You shouldn't have that problem. The center is always the clearest, but with those lenses I wouldn't worry about it.

If you're doing landscape I would shoot at f/8. The lens will perform the best at this aperture. As far as focal length goes… I'm not sure.

The sweet spot is usually 2-3 f stops down. But its a general rule. Its better to read review about the specific lenses to find out where they are the sharpest.

The 'sweet spot' is the aperture that gives best contrast, the lens appears sharper at that aperture.

As already mentioned the 'sweet spot is normally 2 to 3 stops below widest aperture, it has nothing to do with focal length.

That's the 'rule of thumb', but some lenses (notably wide aperture telephoto lenses) are designed so that wide open is the sweet spot because the manufacturer knows that's the reason you bought the lens.

With wide range zooms, there probably isn't a sweet spot as there won't be much variation in contrast at any aperture or zoom setting, its one of the things that gets 'compromised' out of the design, sacrificed to the wide zoom range.