what do you know about purple fringing and how to fight it?
During the last years I have been shooting mainly landscapes and purple fringing is a problem that is annoying be more and more.
Here is an example:
You can see it where the sky is visible between the branches. That is p.f. isn’t it?
Is it similar to chromatic aberrations? Is it caused by the lens? I think it is not quite clear what is causing it. UV light seems to be an important factor though and the fact that digital sensors are more sensitive to UV light than film. Anyway, CA filters in software such as RT and dt do not seem to be able to correct it well. And defringe filters are not optimal either.
Do you have experience with UV filters in this respect and if yes, which would you recommend? Are there any special purple fringing filters?
I have been using UV filters for a long time, but lately I have mostly used no filters. Besides, it is not recommended to use both ND filter and UV filter, is it?
According to my experience, smaller apertures and UV filters seem to help a bit. But my favorite lens is sharpest at f/5.6 or so.
Can ND filters make purple fringing worse?
Could/can it be fixed with software? If yes, how?
I noticed it in your latest playraw. CA didn’t really help. Changing demosaic algorithm from Amaze to VNG4 helped, but still didn’t solve the issue entirely.
Someone can correct me if I’m wrong, but I believe it is clipped highlights (which go magenta) remaining unaffected by the highlight reconstruction algo, probably because they are too close to detail that doesn’t need to be reconstructed (leaves). On your playraw, I found reconstructing in lch gave best results, but only used the default threshold of 1. Playing more with it now, lowering threshold to 0.3 reduces the magenta, and down to 0.1 nearly eliminates it, albeit replacing the magenta with brown fringing (which at least is less conspicuous in foliage). The defringe module does an even better job (increase radius for larger magenta fringes). That combined with lower reconstruction threshold is the best I could do. Still not perfect, but only pixel peepers would see it. I too am interested to hear if there are better methods.
By the way, a better example would be a 100% crop on the magenta fringes or the raw file - I used your latest play raw as example.
ok, thanks for the interesting answer.
You see, defringe does help but it does change the rest of the image, too (usually to desaturation), according to my experience.
Defringe in RT seems to do a better job than in dt, btw.
It is difficult to judge, without the raw. When I take your play raw from the Erlaufschlucht as an example, the fringing appears in regions of the sky that are over exposed. There is not only purple fringing but also a complementary greenish edge. So, this looks to me like chromatic aberratins.
@Thomas_Do
For the above crop, the 5 raw-ca-correction iterations were not really necessary, but for this crop they improved the result (left 1 iteration, right 5 iterations)
I think some modules such as filmic sometimes make fringes worse.
But I wanted to come back to the question what purple fringing actually is any how it is generated.
Only @Soupy said something. So apparently it’s partly clippend highlights. But they cannot be clipped highlights if they are cyan or green.
And the modules chromatic aberrations/lens correction sometimes also make CA worse. In most cases the module chromatic aberration seems to be more effective than lens correction.
I think I must do more tests with a good UV filter and smaller aperture. But I also want to mention that apparently it’s not only the lens but some camera sensors are more sensitive to UV light.
Well, as already said, it is probably difficult to design an ultra-wide lens that is so compact. So compromises are necessary. It is true that it is not a super expensive lens.
As far as raw ca correction is concerned, dt uses an elder version of the algorithm, which does support only one iteration while RT supports up to 5 iterations.
Here’s an example from your latest play raw (top left no raw ca correction, bottom left one iteration (what dt supports), top right two iterations, bottom right 5 iterations)
Many possible reasons for unexpectedly pink/purplish supposedly neutral areas, mostly already mentioned. These are the most common:
In the highlights, Green channel clipped in the raw file but not the other two channels in, say, daylight. If after white balance R and B are higher than G (because G is clipped and remains unchanged), the resulting tone is purplish instead of neutral. In the RT Exposure module you can uncheck ‘Clip out of gamut colors’, check ‘Highlight reconstruction’ and play with the rest of the sliders, including Highlight compression to your liking. In theory this can be fixed ‘perfectly’ if the underlying subject was neutral.
Lateral CA: greenish on one side of a high contrast target like a branch, reddish on the other. The three color planes are out of alignment where this happens because of varying magnification away from the optical axis. You can typically fix the alignment quite accurately by playing with the CA module in RT.
Longitudinal (axial) CA: purplish tinge bleeds into high contrast targets like branches against the sky. The three color channels have slightly different focal planes. Most current solutions are palliative (e.g. ‘defringe’).
It is possible to have all three in a capture. The crop shown by @Thomas_Do shows very obvious strong lateral CA. The small branches seem to be affected by axial CA.
