Chromatic aberration

Chromatic aberration is a lens error found in photography.

Digital ('fake') chromatic aberration

Quite often in early digital cameras (and still - in cheaper models), you could see a purple fringe around certain bright, reflective objects such as chrome etc.. This is not a lens error, but rather a curiosity in the way CCD chips work; The reason behind 'fake' chromatic aberration is not yet certain, but the most viable theory is that it is a refraction on the colour filter layer of the CCD chip.

Chromatic Aberration

Chromatic aberration is the name given to the phenomenon that light passing through a lens is refracted differently, depending on the colour. Remember in high school, when your physics teacher made you shine white light at a lens (well, a prism, but still), so you'd get a rainbow? Exactly. That is an extreme form of chromatic aberration.

Any well made lens can make an extremely sharp picture in a single colour, but the image will be blurred out of focus by all the other colours that come into play. If you plot the focal length of a simple lens as a function of wavelength across the light spectrum, you find that the difference between the minimum (blue) and the maximum focus length (red) is about a half percent of the average focal lenth. This means that a 500mm lens would have a red focal length of 503mm and a blue focal length of 497mm. If you take into consideration that these differences almost make up a centimeter, you realise that this lens would never work.

To counter this, lens manufacturers often use several types of glass in the same lens. Some objectives (achromat objectives, to be specific) are designed from two lens elements, so that the red and the blue falls on the exact same spot. This means that everything in between is slightly off, but when you have the two extremes converging, this is definitely a step in the right direction.

The next step up is to use a triplet objective - or an apochromatic lens. (you might recognize APO from Sigma lens series) This is the same as the achromat objectives, but focuses three colours to the same point. Because all the colours that are recorded in our eyes are red, green and blue, it makes perfect sense to make sure that those three primary colours are focused to the same spot. Guess what? This is exactly what the APO lenses do.

The really top-grade lenses, then, use a combination of apochromatic lens elements and fluorite glass. This, combined with aspheric lens elements and other dingbats (yes, not very scientific, but this is where it goes way over my head), makes sure that as good as all the light comes together where you want it to - rendering the best colour possible.

Source:

A rather interesting conversation with an professor in optics at my university.