In a perfect world, our camera lenses would recreate what we see with perfect accuracy. We’re close, but physics isn’t always that easy to work around. What we often end up dealing with are optical aberrations, which manifest as a number of different imperfections in our photos. Some of them are easy fixes, and others are just part of being a photographer. Here’s an overview of the six main lens aberrations you might encounter.
Chromatic aberration is one of the most well-known and easiest to identify of the bunch. You may have also heard it referred to as “fringing,” a name derived from the way misaligned colors “fringe” around the outlines of subject matter. If your lens is susceptible, you’ll notice it most frequently in high-contrast images. Why does this happen? Chromatic aberration is essentially the result of wavelengths of color failing to reach the correct focal plane. Each wavelength has its own refractive index, and they don’t always meet where they’re supposed to as they pass through your glass.
Chromatic aberration can be broken down two main types, longitudinal and lateral. Longitudinal CA can affect an entire image, and it’s increasingly problematic in super fast primes. It’s the product of wavelengths of colors focusing in different places along the optical axis. Lateral, on the other hand, will primarily affect your corners. This happens when colors all focus on the same plane, but the foci are off-kilter from the optical axis.
Lens manufacturers try to reduce chromatic aberration with special coatings, such as fluorite. Luckily for us, it can also be lessened with stopping down, or in post-processing. Lightroom, for example, has a relatively effective de-fringing tool.
If you shoot a lot of night photography, chances are you’re acquainted with coma. It’s still a challenging issue for modern lens design, though some lenses are much better than others. Coma pulls its name from Latin, where it refers to a cloud of dust surrounding the nucleus of a comet – which is what it usually looks like. This optical phenomenon occurs when off-axis points of light enter the lens and distort, giving the illusion of having tails. They can appear almost as if they’re vibrating, or falling slowly through the image frame.
Fireworks, city lights, and stars are all incredibly prone to this type of aberration. You’ll notice it increasingly worse at the edges of your shots, and as with most aberrations, at a wider aperture. To reduce it, your best bet is to try stopping down. If you’re big into night photography, you may want to borrow or rent a few lenses to see how they handle night shooting. Lenses with aspherical elements are more successful in minimizing this issue.
Spherical aberrations aren’t as much of a problem as they used to be. Thankfully, modern lens design has figured out ways of minimizing them. When you have a sphere-shaped lens, light rays passing through are refracted in different amounts at the center vs. the edges. They end up converging at different points across the optical axis, and try as you may, you won’t be able to focus them sharp. What you end up seeing are points of light throughout the entire image frame engulfed by blurry halos. Like with coma, lenses with aspherical elements best avoid the issue. Their unique shape eliminates the refractive error so all rays meet properly at a single focal point. If you still find yourself dealing with it, shoot with a higher f-stop. A smaller aperture will yield a vast improvement.
Those of you with the eye condition of the same name already know what astigmatism looks like. In simplest terms, it’s what happens when a ray of light entering the lens on a horizontal or vertical plane forms an ellipse instead of a neat focal point. This causes the illusion of directional blur, and makes astigmatism pretty easy to spot. Take a look at the horizontal and vertical lines in an image, especially toward the edges. If you see what looks like inexplicable motion blur, either in a horizontal or vertical direction, your lens is astigmatic. If it’s severe enough, it can be an indicator that your lens (much like an astigmatic eye) is badly misshapen or misaligned. To reduce it, shoot with a smaller aperture.
Some people hesitate to call field curvature an aberration, because it so often can’t be helped. Field curvature is another issue that’s been greatly improved with time and technology, but not eliminated. It causes a perfectly flat object to blur toward the edges of the image frame rather than stay uniformly sharp. This means that even when your lens is perfectly parallel to a flat subject, your corners can appear soft. Lens optics are curved, not flat, so they project light in a curved manner onto our flat image sensors. While the center rays are perfectly focused, the curved rays lose their sharpness toward the periphery. Stopping down can help.
This is another one of the most frequent and annoying optical problems that photographers face. Distortion affects the way rectilinear lines appear in our images. We’ve all seen the bulging effect of barrel distortion when shooting straight lines in wide angle. Telephoto shooters are probably familiar with the opposite, which is the bowed-in appearance of pincushion. Occasionally, but not often, you may come across a combination of the two in a phenomenon known as mustache.
Distortion tends to be more prevalent at the extreme ends of zooms, though primes certainly aren’t immune. Barrel, which is utilized to the max in fisheye lenses, is caused by image magnification decreasing as the subject moves farther from the lens’ optical axis. Conversely, pincushion happens when image magnification increases as the subject moves farther from the optical axis. Since it’s such a common issue, most editing software offers helpful perspective correction tools.