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A couple of days ago I posted an article on the separation of colours to produce the rainbow. I also suggested some application in photography: the use of polarised filters to remove glare and to boost colour saturation.

However, as in most cases, there is a dark side here, too: the creation of chromatic aberration.

We could see how, when changing the transmission means, each colour (or wave length) changed to a slightly different direction.

Lenses...

Well, that effect happens again everytime the means is changing. Now, think of a modern lens, a "standard" zoom. I use the Canon EF-S 18-200mm f3.5-5.6 IS.

According to Canon's specification, it has 16 elements (single lenses) in 12 groups (some lenses are grouped, and they do not move separately). The lens structure, from their brochure, is as follows:

Zoom p

So the light rays change about 20 times the transmission means. Can you imagine what do light rays do? Crazy...

Chromatic aberration

Main problem arises in peripheral areas from the image, where light must flow through through more inclined zones. Refraction effect is stronger, and it is more complicated to "guide" light rays.

Thus, a light spot might produce different casts on the sensor, with slight deviation. If the object has uniform colours, this effect will not be visible, since several points will overlap in the sensor area.

The problem appears when the objects have sharp, contrasted edges, where colour shift can be observed in the sensor. Lightrays displacement, together with separation in the single photo-diodes of the sensor, will record this displacement in final images.

arcoiris 2795pe

Typically, we will see greenish or magenta lines in high-contrast details on peripheral areas of the pictures. 

Can it be ment?

Some SW tools can correct this kind of aberration to some extent. But major improvements in recent years have been made in two main directions: 

Use of aspheric lenses

CAD-design tools have allowed for the design of single lenses without spheric surfaces (for example, parabolic ones), which reduce light deviation in peripheral areas, thus improving lens performance in that regions.

Use of advanced coating

Addition of advanced coating to lens surfaces (some of them based in nanomaterials) reduces the deviation produced upon change of transmission means.

Modern lenses typically include combinations of both aspheric lenses and advanced coating, to reduce aberration as much as possible...

The two above improvements were not habitual in old film lenses; those can of course be used in digital cameras, but you might find that chromatic aberration is stronger in those lenses... 

Tag(s) : #Photography