If only it were so simple. (The cynic in me wants to point out here that if there were anything remotely resembling the best lens, camera or accessory, rest assured we photographers would flock to it in droves, and you wouldn't have to worry about not knowing about it. The same goes for the magic pill that makes us lose weight or regrow hair. But, I digress.)
The truth is, obviously, that there is no universal answer to this question. The answer is different for everyone. But still, how can I help photographers to answer this question that we all have to wrestle with from time to time? Here is the process I use myself, and what I employed with my student: five simple questions to help determine your next perfect lens purchase.
1. Are there features you're especially excited about? This question could also be considered the fun factor. It's good to take into account and you definitely want it to weigh into your decision, but you may not want to base your entire purchase on some tiny little thing you're looking forward to trying—say, a fancy new glass coating, a new and improved focus motor, or a little red paint. But that said, if there are features you're just dying to have in a new lens, you may want to listen to your heart.
Are you itching for a stabilized zoom, for instance, or maybe you're nuts for beautiful bokeh and you've just got to have a 14-bladed aperture. Whatever it may be, if some bit of tech is captivating you, take heed. I find I'm usually happier by going for it—splurging—rather than living with constant regret and unmitigated gear envy. If nothing else, at this point in the process, factor these features in to help narrow down your options.
2. What lenses do you already own? I have a 24-70 and a 70-200mm zoom, both of which I love. I also have a prime 100mm lens of which I'm quite fond. But, I'm pretty hit-and-miss at the wide prime end of the spectrum. Though I covet other lenses—like a super-telephoto and even an ultra-sharp 85mm prime—the lens that's missing from my arsenal is a wide prime, something in the 20 to 30mm range. That's a good place to consider investing in new glass, because it's filling a void in my kit, but it's not the only thing to consider. Will a new lens cover a focal length you've already got in your arsenal? It might still be an important upgrade, because of other things like improved sharpness or added features. That's really where the next question comes in.
3. What's your experience level? Are you a fairly new photographer who is simply looking to try more gear, or have you been shooting long enough (a time frame that's different for everybody) to start seeing the shortcomings of lower quality lenses? If you're fairly new and simply looking to broaden your horizons, you can probably avoid the professional lenses that have superfast apertures, incredible sharpness and high price tags—because you may not yet be able to take full advantage of their sophisticated features and benefits. But, if your goal is to increase the sharpness of your pictures because you're starting to see the flaws from your starter kit, then maybe you need to save your nickels and get ready to make the investment in high-end glass. The point is, typically a newbie shouldn't worry about buying the best, most expensive glass, and someone with more experience shouldn't waste their time with introductory lenses they're more likely to find frustratingly inadequate for their needs.
4. What do you want to do with your new lens? This may seem like a "too obvious" question, but it's also immensely practical. Sure, I have a void at the wide end of my own kit, but it's because I find the 24mm end of my 24-70 zoom takes care of many of my needs. I make an awful lot of portraits, and so a 20 or 24mm prime isn't likely ideal. What might be useful, though, is a 35mm prime. It's still wide, and it's still filling a void for me, but it's also practical for the types of things I will use my lens for—people photos—because a 35mm prime is a great way to offer context in environmental portraiture. In this way, what you want to accomplish can be your guide in finding the perfect new lens. If you're in business with your camera, this question is ultimately going to be what allows you to determine, along with the final question, if purchasing a lens makes good business sense.
5. What's your budget? I know, price is a buzz kill. But, not only is it important in this decision, it's the main way you'll help narrow your viable options. The truth of the matter is, at a certain point in your photographic progression you're going to stop purchasing entry-level lenses in favor of professional glass with professional features, like wide maximum apertures and superfine resolving power. These things don't come cheap, but for serious professionals they're not only worth it, they're necessary. If you don't have the budget for them, it's a painful waste of time to compare the apples you can afford to the expensively unattainable oranges of your dreams. After you've determined what lenses will fill voids in your arsenal, and what you hope to accomplish with a new lens, as well as allowing for the features that are your "must-haves," it's usually your skill level and price point that really start narrowing down your actual choices. For newbies, there may only be a couple of affordable short telephoto primes that fit your camera mount, whether from your camera manufacturer or a third-party maker. Likewise, the experienced pro isn't likely going to upgrade with inexpensive kit glass. If your needs and your budget don't align (the story of my life), then you've got to get creative. Short of panhandling, you could consider renting or buying used, or even pooling gear with a friend. The point is, when buying a lens, it's important to be sure you're working within an honest framework that accounts for your needs as well as your budget; it's the only way to prevent buyer's remorse, whether you're a newbie or an old pro.
If you have your own lens buying advice, please share it in the comments below.