How To Buy A Lens
Before buying a new lens, ask yourself: “What about my current lenses keeps me from making better pictures?” or, alternatively, “Why am I dissatisfied with what I have done so far?” Without a goal, you’re either taking a random shot and hoping it works, or giving in to Gear Acquisition Syndrome; that is, buying stuff for the sake of buying stuff.
If image sharpness is the problem, work on technique and support (monopod/tripod) before upgrading your lens.
If you want more or different focal lengths, ask yourself why you want them. For instance, do you want that ultra-wide lens to “get it all in”? That doesn’t always make for a good picture; ultra-wides do much better when used to get real close to stuff and exaggerate perspective, or when shooting in close quarters (indoor architectural/group shots).
If focus is the problem, first test your understanding of the focus system, then test each lens/camera combination you use for focus errors. Remember that autofocus systems have finite tolerances; there is a range of performance considered acceptable by the manufacturer. If you can’t repeatably generate a focus error under controlled and measured conditions, it’s quite likely you need more practice using the focus system, or your expectations of its performance are unrealistic.
If distortion or chromatic aberration is the problem, there may be no option to buying a different lens if you can’t work around it or correct it in post. If there is a serious optical fault not covered by the above, and the lens is under warranty, it probably needs to be sent in for repair or exchange.
If you want shallow depth-of-field but only have slow (f/3.5 and slower) lenses, a new lens is your only option. There’s no substitute for fast glass when you need it.
Finally, if you’re looking to improve your photography, I highly recommend picking a single lens, preferably a prime lens, and working with it exclusively for an extended time. You will learn its strengths, its weaknesses, and its quirks, and you’ll get used to thinking around its limitations to create the pictures you want. There’s nothing like upgrading the photographer to make your pictures better. 🙂