Chances are that more people have strong opinions about what constitutes a good lens than actually know what separates good from bad.
It almost always boils down to aberration and distortion. I’m going to discuss some of the more common lens problems here in the hope that you can make more informed choices when picking your next lens.
Here are four common lens problems:
This is one of the most commonly-understood lens flaws. It’s caused by dispersion. It is usually manifested as a color fringe. It’s the result of the lens focusing on one single color instead of a range of colors. (There can also be color fringing that evidences itself as part of the digital process. I am not discussing that anomaly here.)
CA is most noticeable in super wide and zoom lenses. The more expensive and quality glass is coated with special materials or constructed with low dispersion glass, most notably, glasses containing fluorite.
This is caused by refraction. Light rays strike near the edge of the lens instead of at the center. This causes a shift in focus. SA is impacted by focus distance and is overcome using floating elements found in higher-quality lenses.
This is caused by changes in image magnification. There are several kinds of distortion, such as barrel and pincushion. The changes in magnification in inexpensive lenses are delivered in waves, making them very hard to correct short of using special software. Lens distortion can be corrected by the use of special coatings and multiple high-quality lens elements aimed at keeping the image magnification constant throughout the light path.
This is caused by cosine effects. Anything from the way a lens is physically constructed to poor glass quality which allows light at the center of the lens to be brighter than at the edges. Falloff usually manifests itself as vignetting. FALLOFF can be corrected easily in most post-processing programs.
There’s no such thing as a perfect lens. Even the most expensive, hand-made, name-brand lenses have imperfections. Most photographers are not capable of getting the high-end lenses to perform at their maximum capability. But many of the very inexpensive lenses do create challenges for photographers that can be quickly solved by moving up a grade or two in lens quality.
The next time you hear a discussion of lens quality, ask the people talking about it what sort of chromatic aberration, spherical aberration, distortion and falloff the lens is exhibiting. You’ll quickly find out whether they actually have a basis for their opinion or they are simply regurgitating something they saw in a camera club forum.