Still photography is a complex blend of applied technology and artistic creativity. Regardless of which direction one leans—and most photographers tend to lean at least a little bit in one direction or the other—it’s hard to make stand-out images without using both artistry and craft. Of course each person’s objectives differ and personal predilections and subject matter help determine the selection of tools most suited for the job.
On the tool side of the ledger, lenses are (arguably) the most important link in the imaging chain. They are the components that focus light on film or sensor and determine to a large degree how well the photographer’s vision will be expressed. Of course lenses come in immense variety—fast, slow; long, short; heavy, light; relatively inexpensive, expensive beyond belief; made cheaply, manufactured to the finest tolerances possible; etc. But no lens is perfect, regardless of who makes it or what the price may be…it’s just not possible.
All of us have either experienced or heard about “good copy” and “bad copy” lenses. The implication is much like getting a “lemon” in the automobile business—the item just doesn’t function as it’s supposed to. To be sure, this happens once in a while, even in the best of circumstances, but perhaps not as often as one might think. For a very thoughtful and detailed discussion on this topic have a read of the article by Roger Cicala on DP Review. I’ll warn you—this will appeal more to those who enjoy the tool side of the equation as it gets quite technical with topics like variation, Subjective Quality Factor, and MTF charts. But the summary is instructive. In a nutshell: variations occur, but are usually not significant; small variations simply aren’t visible most of the time in real life; once in a great while variations can combine with a specific lens and specific body in a negatively observable fashion; in rare cases a really “bad” lens gets through but it’s easy to detect; and camera autofocus is less accurate and more variable than one might think.
Put it all together and it ought to help reduce anxiety about lenses in general. My suggestion is to assess your objectives and your budget, purchase a few quality lenses, and make a lot of images with them. That way you’ll be spending more time on applying artistry to your work and less time on tool obsession and chances are you’ll like the results.