Lens Aberrations

In our last post, we looked at how aberrations are the root cause of the use of multiple lenses in constructing a complete camera lens. Today, and over the course of future posts, let us consider in more detail the concept of aberration. 

Most generally, aberration refers to the distortion of an image when passed through an optical system, such as digital camera lenses. Limitations of the system might lead to blurring. A common misconception about aberrations, however, is that they are caused by imperfections in the device or system. Rather, the real root cause of aberrations is in fact in the relatively simplistic and, therefore somewhat inaccurate, boundaries set by paraxial theory--that theory around which optical systems are built. To put it more succinctly, even lenses built perfectly to the specifications of paraxial theory will experience aberrations because the theory does not perfectly describe the behavior of light in such systems.

Aberrations can be broken down into two major categories. Those derived from geometrical structure and those derived from the effects on wavelength due to dispersion. Though shape and material are important to both categories, the shape will obviously play a larger role in the former, causing blurring, and material/density will play a larger role in the latter, causing shifts in coloration in addition to blurring. Geometrically derived aberrations are called monochromatic because they affect even monochromatic light, whereas dispersion-based aberrations are called chromatic because they are only evident in situations with multiple color variants.

Digital camera lenses correct for these aberrations and multiple subcategories therein. Soon, we will take a look at some of the ideas underlying the most common chromatic and monochromatic aberrations. 

A Lens Is More Than a Lens

The basic principle behind a lens is simple: the device mimics the eye, capturing and ushering light toward a processing center, one that is either immediate (digital) or delayed (chemical and film). Yet, the actual physical properties of light and the shape of a lens make the situation slightly more complicated.

First off, a camera lens worth its salt is rarely in fact just one lens. We call it “a lens” out of habit, but in most cases it is in fact a conglomeration of many lenses, referred to most commonly as lens elements. “Lens elements” as a phrase is itself misleading. In fact, the double meaning of the term "lens," referring to a camera part, and "optical lens," referring to any lens used for a variety of purposes, lends itself to a kind of linguistic and lexical problem: when referring to the parts that make up the whole, both have the same name. Suffice it to say that most camera lenses are not lenses at all in the most traditional sense, but rather are themselves a series of optical lenses collected together to make a more complex and precise device.

Why are there multiple lenses in digital camera lenses? The simple answer is that a single lens has what is termed “aberrations.” These are areas of the lens where light becomes distorted, either losing focus or distorting coloration, and in fact a host of other issues. The multiple lenses serve to correct these aberrations.