Is there a notable difference in sharpness between aperture stops of a lens?
If so, where can I find that information for my Canon 550d Kit lenses?
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All (well, all good) lenses are desiged to have a sweet spot, often around f7 - f10.
VERY wide lenses such as my 50mm f1.4 suffer from signifficant "bloom" at wide appetures in high contrast areas when wide open.
When closed right down there will also be a degredation in quality.
See the images below:
Left f10, middle f5.6, right f1.4
And that's on a Nikon D800 with a Nikon 50mm 1.4G (NOT cheap kit!)
As for your Canon kit lenses im not really sure that information is published to be honest - it only takes a few minutes to play about with it and find out for yourself :-)
Yes. Generally lenses are at their worst wide open or stopped right down. The former is especially the case when it comes to cheaper zoom lenses. Most lenses have a 'sweet spot' where quality is best - an f4-5.6 lens usually has a sweet spot around f8-11. A relatively cheap 1.8 prime usually has a sweet spot around 2-3.5
More expensive lenses are generally much better wide open - it's one of the reasons they're more expensive.
Regardless of the lens quality, they will all suffer a loss of sharpness at the higher f-stops (f16 and up) due to diffraction - see this question for more on that.