Chromatic Aberration can be seen as color fringes, most noticeable near the edge of an image. This is caused by different wavelengths (colors) of light being focused at different points.
In the image above you can see chromatic aberration - a green fringe on the left of the leaves, and a magenta fringe on the right.
This happens because lenses have a different refractive index for different wavelengths of light. So as the light passes through the lens, it can become split, like it does through a prism. The problem is only relatively small, but it can show up in images, particularly if you view them at large sizes.
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