The image above is an extreme example of what you don’t want to see in a new lens, or any lens for that matter. A lot of people will buy a lens on line to get the best price and most of the time you will get a good lens. Now assuming the lens you just purchased is perfect, why are your images looking less than perfect? Lens making is measured in millionths of an inch and machining is measured in thousandths of an inch. If the camera body and lens were machined to perfection, your camera will have the ability to produce sharp images. However, Manufactures have a plus and minus tolerance range and anything with in that range is considered acceptable. Lets say that you have a camera that is rated a minus 2 within the acceptable range and your lens is rated a plus 2, the result is a perfect match. On the other hand if your camera is a minus 2 and the lens is a minus 2, your images are going to be a little out of focus. Prior to my knowledge of this information, I would judge a lens quality based only on how it worked on my camera. When I was switching camera systems, the salesperson suggested that I try one of the aftermarket lenses. After comparing several test shots with each lens, I ended up purchasing two aftermarket lenses. Were the camera manufactures pro lenses bad, probably not, but they were not sharp on my camera. Not too long ago, the only fix was to send the lens and camera to the manufacture to have them adjusted. Most pro cameras today are set-up so you can make your own adjustments. This flaw occurs when, the lens is either front focusing, or back focusing. Personally I prefer to always try the lens at the store and carefully check it on my laptop. One other lens problem that I have run into, was a wide angle lens from the film era. It was a 28mm shift lens that worked great, until I moved to a full frame sensor. It still captured sharp images, until the lens was shifted. Digital sensors need the light rays to strike the sensor pixels as straight as possible. Light coming at too strong of an angle loses image quality.