Purple fringing revelation

[Mike_Bing]

On some of my lenses, purple fringing wide open and on contrasty backgrounds can really ruin images. Some would tell me to choose a smaller aperture to avoid this but that’s not always good if the extreme wide aperture is the very reason you bought the lens.

A good example is the Sigma 85mm/f1.4 (dubbed “the Beast” in my house because of its size). It has been with me since my K20D days and apart from having to use AF finetuning for it on every Pentax body I owned since, it delivers beautiful images only marred by two things: extremely busy and distracting bokeh (only under certain circumstances) and really, REALLY bad purple fringing. The busy bokeh I cannot do much about until this day - it is something that bites me now and then.

The purple fringing was bad on the K-5 IIS, the KP after it and my current K-1 MkII to the point I only used this lens for non-landscape shooting. Every purple fringe removal tool in the past ended up doing two things: 1. remove the fringe and 2. Remove some of the detail and/or proper color around it and in the background.

I’ve been using lots Darktable, Gimp with GMIC and Digikam for over 15 years as main editing tools and I have no idea how I have overlooked unpurple by Martin Jambon. It was quietly ported and included into GMIC by Stanislav Paskalev and had totally escaped my attention until recently.

By trial and error (and using the “copy GMIC command to clipboard” button in GMIC_QT) I found settings I liked.

They do not noticably desaturate the sky behind some purple tree branches for instance but have a radical impact on the fringing.

Here’s the bash script (I’m no coder but I have a basic script structure that I plug GMIC command structure into) which takes any number of input images, defringes them and outputs them with “-unpurple” added to the file name.

#!/bin/bash
let "nbfiles = $#"
ref=$(kdialog --title "Purple fringe remover" --progressbar "(Initialising ...)" 15)
qdbus $ref Set org.kde.kdialog.ProgressDialog maximum $nbfiles
compteur=0
for i in "$@";do
	if [ -f "$i" ];then
            let "compteur +=1"
            FILE="$i"
	fi
    FILE_NO_EXT=${i%.*}

    echo $i
    echo $local

    gmic -i $i unpurple 1,0,0.5,0.5,5,0,"0",0,50,50 -o $FILE_NO_EXT-unpurple.jpg

    exiftool -overwrite_original -TagsFromFile $FILE $FILE_NO_EXT-unpurple.jpg

    qdbus $ref setLabelText "Unpurpled image created: `basename "$FILE"` ($compteur of $nbfiles files)"
    qdbus $ref Set org.kde.kdialog.ProgressDialog value $compteur
done
qdbus $ref close

Example before:

and after

[Terry (Terry Pinfold)]

Thanks for sharing this information. In DT I find color equalizer or color zones effective for purple fringing. Your example is extreme. I usually only use the lavender slider in color equalizer, but I needed to use the magenta slider on as well for your image.

I presume your GIMP method does something similar.

[paperdigits (Mica)]

Nice that you found a gmic command!

Does this lens have a profile in lensfun? That might help.

[Mike_Bing]

It does but specifically with this lens it doesn’t take away all of the purple fringing, quite frankly it does a terrible job at apertures between f1.4 and f2.8. My experience with the lensfun library for many years is that it is really good for distortion but both TCA as well as vignetting definitions are often imprecise.

The TCA correction line in lensfun for the Sigma 85mm/f1.4 seems to be somewhat of a compromise value shared across all apertures whereas lenses that were better defined have various correction values for different apertures.

The second reason is that the specific algorithm used in GMIC seems to take more things in consideration, specifically edge detection, and can therefore address fringing more aggressively without damaging other parts of the image.

[Thomas_Do (Thomas)]

In your example, there are also blue CAs. Any plan to tackle those too?

[Mike_Bing]

As long as they don’t bother me too much, no. The purple really stands out usually and can destroy an otherwise perfectly acceptable image. Can you point me to an area where you see blue CA? I’ll take a closer look. Thanks!

[Thomas_Do (Thomas)]

@Mike_Bing

Can you point me to an area where you see blue CA?

Here?

[Terry (Terry Pinfold)]

@Thomas_Do

there are also blue CAs.

The method I showed with color equalizer can tackle this problem the same as purple fringes. You might limit the effect to branches by using a brush for masking.