What Makes a Lens a Good Lens?
Everybody wants the best lens they can, right? If only we could agree on just what that meant.
One obvious criterion for a good lens is that it is sharp. The whole point of a lens is that it takes sharp images that can do justice to the megapixels we paid for when we bought our cameras. Sites such as DxO Mark publish test results that rate lenses in terms of sharpness and lack of optical distortion. Given that they have better test gear than do most of us, I appreciate the work it takes to make this data available. But sharpness isn't the only way to judge a good lens.
Photographers also tend to covet lenses with fast maximum apertures. Wide-open apertures can render the shallow depths of field that isolates a subject against its background. Portrait photographers have traditionally loved lenses with wide apertures. Lately, photographers shooting the night sky love fast lenses even more, not to isolate their subjects, but to record them at all. A fast lens lets you take pictures of the Milky Way and other night star patterns not otherwise possible. Yes, you could just set the shutter speed to an even longer time to allow the use of a slower lens, but then the movement of the stars would show in the image. They make it all the way around the earth over the course of a single day, and their steady progress will show up in your images if you let it. Then again, you could use a motorized mount to track with the stars so they remain as point sources of light, but then the horizon and any terrestrial objects would show movement in the opposite direction.
Build quality can be a concern for some. If you only shoot an occasional image with a given lens and treat it well, you may not mind a less robust lens, but if your livelihood depends on it or you are rough on your gear, you need a lens that is built to last. As a generalization, third-party lenses get a bad rap for their build quality, but there are exceptions. Many years ago, the best lens bodies were constructed completely out of metal. As lens construction began using various space-age plastics to save weight, not everyone was happy. Never mind that metal can dent, while plastic can bounce right back, presuming it doesn't break of course. Over time, most lens makers adopted the use of plastics for at least some components, and when used judiciously, build quality doesn't appear to have suffered.
Photographers often want the best lens they can without spending a lot of money. Cheaper price tags can be temping indeed. Sometimes you get what you pay for, but prices do vary widely if that were your only criterion, even within a single brand. Third-party lens makers' business model relies heavily on producing lenses that have similar specs to the name brands but at a lower price point. That sounds good. Or save up your money and wait for the next rebate or sale that happens. Saving money is always a good thing. Or at least a tempting one.
Lightweight lenses tend to be preferred over heavier models, other things being equal. When you're carrying all your gear on your back, every ounce saved can help. There's always a temptation though to load up with yet another lens when you find yourself with a lighter load, but that could be a good thing too, if it means you can get shots not possible without that extra lens. Or maybe you can bring along something more extravagant than usual for lunch. That could be a tasty way to make use of your weight savings.
Focal length could be a determining factor too. For the purposes of this article, I have generally assumed were talking about a good lens for a given reach, but it's worth acknowledging that its possible to find yourself shopping for a lens without any firm commitment in terms of focal length. Or maybe you're hankering after a new zoom but are open as to how wide the range should be. Or perhaps, like me, you've developed an unquenchable thirst for ever-wider lenses. Wide maybe good, but wider is better.
Other, non-optical features might make one lens seem better than another. Lenses with vibration reduction are easier to hand-hold than those without. Some photographers are perpetually in search of lenses with the best bokeh possible. When looking for longer focal length lenses, I would definitely prefer one with a built-in lens collar, both to facilitate switching from portrait to landscape orientation and to balance the weight more evenly over the center of the tripod head.
But wider lenses or extended range zooms tend to have at least minor optical defects when compared with more mainstream designs. And lenses with fast maximum apertures tend to cost more and weigh more than slower versions. Most of the factors I've listed in describing a good lens have an impact on most of the other factors. Try as you might, something's got to give. It's simply not possible to optimize all these variables in any given candidate lens. You can try, but it's gonna get awfully complicated awfully fast.
It's no wonder people often argue about which lens is best. When it comes right down to it, even with all the objective data possible, it's up to you to decide which of these criteria you care about the most based on your own circumstances and needs.