I'm new to photography, and am reading a lot about how people will pay thousands of dollars for a lens because its really high quality and produces superior images. What though is different between the really expensive high-end lens and a cheap one in terms of image quality? I understand expensive lens are probably made better and thus are more durable and will last years longer. However, in terms of just the image the lens will produce, is a high end lens any better than a cheap one? If the image is better, what makes it that way, the glass itself in the lens?