Chromatic Aberration

Chromatic Aberration

OK, physics time. C is for chromatic aberration...

You know that classic Pink Floyd album cover, with the prism splitting light into a rainbow?

Well, you can think of a lens as being a series of prisms, and so they bend the light, and (here’s the thing) bend different colours to different degrees. Blue light comes to a focus closer to the lens than red light.

With early cameras and plates that weren’t colour-sensitive, this was a problem, because focussing by eye uses predominantly green light, while early emulsions were sensitive to blue and ultra-violet light.

The solution was to use a combination of lenses, made of different types of glass. Different glasses bend light by different amounts, and also separate the colours more or less. While the maths is utterly beyond me, I get the idea that you can make problems cancel each other out by careful choice of the characteristics of the glass, and the curves of the elements.

A lens corrected so that green and blue-violet light are in focus in the same plane is called an achromat. As photographic emulsions that are sensitive to green and then red light were developed, even more correction was required, and this gave apochromatic lenses.

But nothing works perfectly, and so you may find that there are still small problems under difficult conditions. Usually, this shows as green and purple fringing of fine detail against the light. Now, my Ilford Manual tells me that this is, strictly, not quite the same thing, being the result of light of different colours forming images that are different sizes, rather than at different distances from the lens. It’s called lateral colour. The image below was shot with a Minolta 85mm f/1.4 lens, at full aperture, and is from near the corner of the frame. Not at all a bad lens - but not perfect...




The root lies in the same thing, though – the fact that all light is not equal in the lenses of your camera.