Purple Fringing on Oly and Pany Bodies: Causes and Remedies

Started Jun 7, 2013 | Discussions thread
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Anders W
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Purple Fringing on Oly and Pany Bodies: Causes and Remedies
Jun 7, 2013

As many forum members are already aware, images produced with MFT equipment sometimes show so-called purple fringing (PF). In some respects, this is not a phenomenon specific to MFT but one that can appear with any camera system. One example, caused by longitudingal chromatic aberration (LoCA), is PF in the out-of-focus area along contrast edges of objects closer to the camera than those in focus. Another example, again caused by LoCA, is PF in the in-focus area when fast lenses are used at wide apertures. This second type of PF, however, is not very common with native MFT lenses since most of them are well corrected optically in this regard.

However, there are also some manifestations of PF that are possibly more MFT-specific. To begin with, Panasonic lenses are designed to be software-corrected for lateral chromatic aberration (LaCA). When such lenses are used on Olympus bodies, the software correction for which the lenses are designed is not carried out, which can result in rather prominent purple or magenta fringes on one side of an object and green fringes on the other. Even some Olympus lenses can show quite a bit of this phenomenon in spite of the fact that they are presumably designed to be optically corrected for LaCA (since no Olympus body autocorrects for the problem, no matter which lens is used).

Now this problem can be solved by using a RAW converter (or other PP tool) that can correct for LaCA, such as Lightroom (LR) 4.x, which contains a checkbox labeled "Remove Chromatic Aberration" intended for that purpose. As at least some of us have observed, however, checking this box does not always remove the fringing completely. The green fringe is gone but some of the purple remains and can only be eliminated by other means, such as the defringe tool of LR 4.x, whose presence is certainly welcome but whose use is not completely free from downsides.

This residual PF is a bit mystical in several respects. First, it appears on one side of an object only, not both sides as would be expected if it were caused by LoCA. Second, it is more prominent at the edges of the frame than in the center, not equally prominent everywhere as would be expected if it were due to LoCA. Finally, it is only marginally reduced by stopping down, not strongly so as would be expectued if LoCA were the culprit. So what's actually going on here?

Below I show five 100 percent crops, all from images shot with the Pansonic 20/1.7 wide open and all processed by means of LR 4.4.

The first is shot with the G1. This means that LR corrects the LaCA regardless of whether its LaCA box is checked or not. As you can see, there are hardly any traces of PF in this case.

The second and third crop are both from the same E-M5 image, just processed differently. The LaCA box was left unchecked for the first of the two but checked for the second. As you can see, the residual PF is in this case quite prominent, not only when the box is left unchecked but also when it is checked.

The two last crops are again from the same E-M5 image, just processed differently in the same way as the previous pair. The image used, however, is not identical to the one used for the previous pair. As you can see, the residual PF is in this case far less prominent. With the LaCA box checked, the result is now nearly as free from PF as the crop from the G1 image.

Before I try to explain what's going on here, a little guessing game: What is the reason why the E-M5 leaves residual PF even when LaCA is corrected while the G1 doesn't, what "secret anti-PF weapon" did I use to pretty much eliminate that residual PF, and what explains its effect?

Everyone is free to guess except "secretworld" who, as his nick suggests, has privileged information on all the secrets of the world.

Crop 1: G1

Crop 2: E-M5, LaCA box unchecked

Crop 3: E-M5, LaCA box checked

Crop 4: E-M5 with "secret anti-PF weapon" and LaCA box unchecked

Crop 5: E-M5 with "secret anti-PF weapon" and LaCA box checked

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