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After shooting several images with the Samyang 12mm f/2 for mirrorless APS-C, I noticed it produces a quite strong color cast. It looks like green vignetting. As far as I know, this isn't uncommon for wide-angle lenses.

I experimented with radial filters in Lightroom, but the results weren't satisfying. Is there a method or tool to consistently remove this cast?

Here is an example taken at daylight, ISO 200 and f 5.6, which isn't even wide open. I cranked up saturation to make the defect more apparent.

wide-angle color cast, enhanced saturation

Here is the same image without editing:

unedited wide-angle color cast

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  • \$\begingroup\$ Did you try turning on Lens Corrections? \$\endgroup\$
    – Mike Dixon
    Sep 12, 2017 at 12:06
  • \$\begingroup\$ Yes, but Lightroom didn't have a matching profile. You are right, I should have a look wether there is one available, but still, as far as I know (I just tried with some other profiles), lens correction only affects distortion and vignetting, not color problems. \$\endgroup\$
    – smow
    Sep 12, 2017 at 12:30
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    \$\begingroup\$ This would be easier to answer if we could see the image without your cranked up saturation. \$\endgroup\$
    – StephenG - Help Ukraine
    Sep 13, 2017 at 3:09

3 Answers 3

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Try taking a picture of an evenly lit white background - it doesn't have to be in focus (and might possibly be better with MF set to your usual subject distance, in case the effect changes with focal distance). If anything, out of focus is probably better, since any marks on your background will be blurred.

Then you can subtract the resulting colour cast image from your photos in post processing. or invert it and add; not sure what combination mode will work best; perhaps multiply with the inverted colour cast image.

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  • \$\begingroup\$ I tried this, had two issues: \$\endgroup\$
    – smow
    Sep 13, 2017 at 12:37
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    \$\begingroup\$ 1) funny enough, my bias picture ( dropbox.com/s/ankt3fip1y5gs5t/_9130048-2.jpg?dl=0 ) has a vignette and a colored edge, but it is not green, but violet/blue! No idea how to explain this... 2) adding/substracting inverted/using "color" mode at 50% layer opacity... nothing realy worked to even out the image, there is always a ring of color left between the center and the edge. Might have something to do with linear vs logarithmic addition of the color levels, but that is just a wild guess. \$\endgroup\$
    – smow
    Sep 13, 2017 at 12:44
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    \$\begingroup\$ It ought to be a colour equivalent of using flats in astrophotography - there, you take a similar reference image through your telescope, and the stacking software uses it to compensate for uneven illumination and dust bunnies - basically, it knows the reference image value at each pixel, works out a scaling factor for each pixel to boost that to the highest value, and then scales the real image accordingly - to give you the equivalent of an evenly exposed image. What you want to do is the same thing on a per-channel basis. \$\endgroup\$
    – JerryTheC
    Sep 13, 2017 at 22:03
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    \$\begingroup\$ You could possibly try splitting your image into three monochrome ones (one for each channel), do the same with your reference image, and try feeding each pair through the free deep sky stacker, then recombine the results. But that's a horribly tedious way to do it regularly - though it might show whether the idea works. \$\endgroup\$
    – JerryTheC
    Sep 13, 2017 at 22:07
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To correct vignetting and color cast, I use RawTherapee with flat-field correction.

For each lens and each aperture, I take a white picture by photographing against the sky with a white tissue or white balance filter and overexposing 2 stops. Here are before and after images, in which you can see that the vignetting and blue cast are corrected:

before after flat-field correction

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After a lot of searching I found some tools that do exactly what I needed:

Both Cornerfix and the DNG Flat Field PlugIn work direktly (and only) on DNG files, are easy to use and produce good results; batch processing is possible.

All these tools work the same way (as JerryTheC suggested): You take a "Flat Field Image", which is an evenly illuminated and uniformly white image. Some suggest shooting a white wall; I found it much easier to cover the lens with a white piece of plastic (a kitchen cutting board, in my case). It helps to set the focus to infinity so that the white filter is out of focus.

The tools then use the information of this image to cancel out any brightness falloff or color cast to the image corners, even dust bunnies on the sensor or lense can sometimes be removed with this.

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  • \$\begingroup\$ I posted this since it is the answer to my original question; however I actually still couldn't reproduce how I managed to create the green edges seen on the original image. Every flat field I made (tried through all different apertures and focus distances on the lens, also many other on camera settings) produced a color cast in the image corners that was definitely purple, never green... \$\endgroup\$
    – smow
    Sep 15, 2017 at 16:23

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