All lenses have a component called a diaphragm built into them. The diaphragm controls how much light leaves the lens towards the camera's sensor by changing the size of a hole (called
the aperture) in its centre:
When any lens is being designed, some compromises are made and the result is that for a given lens there will be an aperture where the lens operates at it "best". To find out where the sweet spot
is on your lens, take a photograph of the same thing at each aperture - ideally, use a tripod. I've done that of a telegraph pole with my 80-300mm lens:
You'll be pleased to see I haven't included every photograph, but the conclusion was clear - my lens has a sweet spot at f10 - in that image, the foreground looks crisp,
with good contrast and the background looks smoothly out of focus. Now when I use that lens, I know that f10 will give the best images.