A Slight Aberration
My dictionary defines "aberration" as an unwelcome departure from the usual or expected, but the term has a more specific reference in photography. It means those annoying color fringes and visual distortions that some lenses sometimes produce. Knowing the enemy is the first step toward conquering it.
Photography lenses are capable of magnificent feats. To properly focus an image onto our sensors, they bend and shape incoming light with utmost precision. But no lens can be perfect since we're dealing with physical properties in the real world. As a kid, I loved playing with magnifying glasses, not to incinerate passing ants as if I were Godzilla with my heat ray, but because I loved everything to do with science. It never bothered me that the image was sharp in the center and fuzzier around the sides. But these days, I expect more from my camera lenses. I think most of us would.
When a lens doesn't perform as it theoretically might in a perfect world, we can blame the limitations of lens construction. A single lens element like my childhood ant tormenter ("scientific instrument") will inherently have aberrations. The glass has to be curved to focus light, but light passing at an angle through a surface will refract. Refraction is responsible for the beautiful rainbow patterns from a prism. But no matter how pretty, I'd prefer not to see them haphazardly present in my photographs. All sorts of things can happen to light as it travels through a lens, causing blurring, light fall-off, and a range of similar maladies. These are what we call aberrations.
In their efforts to remedy such problems, lens makers construct their products using multiple pieces of glass of various types combined to minimize the chances of aberration. An ordinary, fixed-focal length lens may only need a few glass elements to do its job, but it's not uncommon for long-range zooms, ultra-wide, and other specialized types to have ten to fifteen. A few have even more. But more glass makes lenses cost and weigh more, so it becomes a matter of trade-offs like many things.
One common trade-off is known as chromatic aberration. As light diffracts through the lens, it moves at a different speed than through air, causing the colors to spread like a rainbow. You can see chromatic aberration as purple fringes on one edge of strongly back-lit objects and green rims on the opposite side. You can see it most commonly around the outer portions of the frame, and in some cases, you can see it quite easily. There's little you can do to avoid chromatic aberration other than to minimize the contrast with the background or, of course, to invest in a better lens. But while perhaps tedious, you can reduce its impact by selectively desaturating the fringes, so the color doesn't show as much. Even if you have to tone them down to pure gray scale, you will be better off than you would with purple and green peeking out all over. Once you see it, uncorrected chromatic aberration is hard to unsee.
Other types of lens aberration are generally less dramatic in appearance but can be just as detrimental in their impact. Most of us have seen vignetting when the corners of the frame become darkened. As light passes through the lens, some may become blocked by the supports holding all those glass elements in place or even by the lens barrel itself. When it does, vignetting happens. Switching to a focal length closer to the middle of the zoom range can often clear up the problem, as can a change in aperture. It's one thing to add a vignette intentionally to emphasize your subject. But it's something else again to deal with one, whether you want it or not.
Spherical and coma aberration can result from the increasing curvature of lens elements as you approach the margins. Light entering here becomes diffracted more strongly than light entering more on the lens axis in the center. This variable diffraction can be seen as a soft-focus effect, either in every direction (spherical) or in primarily a single dimension (coma aberration). Both will be more present around the edges of the frame.
Projecting an image through a curved lens element onto a flat sensor can cause barrel distortion if not adequately corrected by the lens manufacturer. You can see it in lines that should be straight, appearing bowed outward. The opposite, known as pincushion distortion, makes everything look squeezed inward. The extreme closeup perspective from shooting with a wide-angle or fisheye lens can exhibit similar phenomena. But in this case, it stems more from the unusual perspective than from a defect of the lens. "Rectilinear" wide-angle lenses can correct for much of this optically. But it's more of a feature than a bug when shooting with a fisheye. Objects at the edges of the frame are so exaggeratively further away than in the center when.
While ordinary lens flare is sometimes considered a separate category, you can also view it as aberration since they aren't supposed to be there. The best way to avoid lens flare is to keep your lens clean and use a lens hood. If that doesn't do the trick, you might be able to shade the glare further with your hat or another handy object. Just extend your arm and wave it around until you find the spot where it creates an artificial eclipse.
Modern software can correct some of these with varying degrees of success. DxO Optics made its reputation on providing just this capability. Assuming they make a module specific to your particular camera and lens combination, it can work wonders. I've long had a love/hate relationship with DxO software since their user interface has always been somewhat clunky, and it doesn't fit in well with Lightroom. But it's handy to have around when you need it.
But with aberration, your best bet is to avoid being bitten by it during capture, so you don't have to attempt a cure with software when you get back home.