Chromatic aberration is an issue which happens when a lens fails to bring all wavelengths of color to a focus at the same point. It can occur with lenses in cameras, microscopes, and optics devices such as binoculars, and it can also be an issue for the human eye. When chromatic aberration occurs, objects can appear blurred, and halos and streaks of color may appear in the image. There are two types of chromatic aberration: longitudinal and lateral, also known as transverse.
This issue occurs because of the differing wavelengths of visual light. When the different lengths pass through a lens, they are refracted differently, causing blurring because they are not brought to a single focal point by the lens. For example, the edge of an object may appear fringed with color as a result of lateral chromatic aberration, and longitudinal chromatic aberrations can make it difficult to track objects and keep them in focus.
People have been recognizing this issue when working with lenses for hundreds of years. As people learned more about the nature of visible light, they began to take steps to reduce chromatic aberration, including developing achromatic lenses which would correct for refraction errors to keep the focus tight. Such lenses can consist of several lenses in a set, or a single lens which is specially cut to address issues with focusing color. Within the eye, the structure of the eye is designed, in part, to correct for chromatic aberration so that people can see clearly.
Chromatic aberration is the bugbear of many photographers. It can be a large problem with wide angle lenses, and it can crop up near the edges of an image, even with very high quality lenses. Many photo editing programs are equipped with tools which can be used to correct chromatic aberration in post production, which illustrates how common this issue is. One thing to be aware of is that while chromatic aberration may not be readily apparent when an image is viewed at a small size, it can be very noticeable at larger ones, making it a good idea to magnify an image to confirm that it is crisp before making enlarged prints.
Problems with color registration and focus are not always caused by chromatic aberration. There may be other issues involved, such as lens flare, or even problems with the storage method, whether it be film or a memory card.