There are two forms of chromatic aberration – lateral and longitudinal. Lateral CA is caused by a difference in how colors are refracted (and thus appears at edges of images) and is easily corrected in post. Longitudinal CA is caused by colors focusing on different planes and appears equally over the entire image. It is also more difficult to correct.

The fringing you are seeing is the result of longitudinal CA. Ideally you want the AF system to focus perfectly at the center of the visible spectrum (i.e. green). At wide apertures this will result in red and blue channels being slightly out of focus in approximately equal amounts – thus you get the magenta, or purple fringing. Stopping down will largely eliminate this.

I think FoCal is handling this exactly right. When focus is “perfect” you will see the most apparent fringing. Even going slightly out of focus will make the appearance of fringing go away because the difference in focusing for the three colors (RGB) will no longer be apparent because all three colors are slightly out of focus.

In other words - when focus is nailed your camera will record, with maximum fidelity, exactly what your lens can produce - warts and all. With fast lenses wide open, that means reproducing longitudinal CA.

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