Chromatic abberation

Photo by Robert F. Tobler

If you have ever looked through cheap children's binoculars the effect of chromatic aberrations will be all too familiar to you.

This “purple fringing” can sometimes be found on digital cameras as well. To what extent it becomes visible is dependent on the sort of image and the presence of dark or light edges in it.

Different wavelengths of light have different focal lengths and chromatic aberrations develop because of the camera's lens inability to focus these different wavelengths of light onto exactly the same focal plane.

Often the effect of chromatic aberrations is amplified by what we call blooming – the overflow of charge from one pixel to its neighboring pixel on the camera's sensor.

Chromatic aberrations are reduced if special lens systems such as achromatic or apochromatic doublets are used. These use two or more pieces of glass with different reflective characteristics. Not even these are completely perfect however.

In your digital darkroom it is possible to reduce the saturation of the specific magenta hue that causes the "purple fringing". Although this doesn't completely solve the problem it certainly makes it less visible as it replaces the magenta with a shade of gray which is far less prominent when looking at the image.

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