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Axial aberration

Axial aberration, or also called longitudinal aberration is an artifact that occurs depending on the focus distance. It inverts the chromatic aberration depending on if it is in the near, or far field. Most often this is a combination of purple and green.

Example image of effect

Notice the aberration that changes depending on the near and far field.

Axial aberration green purple Axial aberration blue yellow Axial aberration red blue
All axial aberration settings: green-purple (default) blue-yellow and red-blue. Chart used in this render: http://regex.info/blog/photo-tech/focus-chart

Why does it occur?

Below is a video that shows the light ray entering the lens. But each light frequency is hitting the focal point at a different angle. This causes the chromatic aberration to change depending on the focus point. Also notice how the light rays inverse in the near field and the far field.

What is far and near field?

Near field is after the individual lines have collided (closer to the sensor), far field is before they have collided (closer to the lens).

What does the axial aberration amount knob do?

It sets the angle light rays enters the lens: