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Explaining chromatic aberration

By July 24, 2018 Photography No Comments

No matter how much you spend, or which brand you invest in, no lens is truly perfect (although some come exceptionally close).

Designing a lens is all about overcoming problems in an attempt to get the light from the scene to the sensor as purely as possible. To achieve this, high-end photographic lenses consist of multiple glass elements made from different types of optical glass and treated with a variety of coatings, each designed to minimise or negate various defects. But, problems still arise, such as chromatic aberration.

White light is made up of a range of wavelengths, from red to violet. The classic way of demonstrating this is to pass white light through a prism and watch as a rainbow of colours emerges.

The same thing also happens as light passes through a camera lens. Although not as pronounced, the glass elements in a lens change the angle of different wavelengths by different amounts. As a result, the red wavelengths might exit the lens at a different angle to the blue wavelengths, and the two won’t be focused on the sensor at the same point. This misalignment will result in coloured fringes appearing around elements in the image (especially high-contrast edges toward the corners of the frame).

Chromatic aberration is most often seen along high-contrast elements in a scene, especially toward the edges of the frame where the light rays have to be ‘bent’ more to pass through the lens.

To minimise this, camera lenses use elements made out of different types of optical glass. Each element affects the wavelengths of light in a different way, with the overall aim being to bring all the wavelengths to focus at the same point on the sensor. However, this doesn’t always happen perfectly, especially with extreme wide-angle lenses and zoom lenses that cover an expansive focal length range.

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