Yesterday (dec 2016) when I played with affinity photo I've encountered a nice technique to kill unwanted
lightning in images... Espcially if  you want to make them seamless.

This technique does not apply to all images, and since the color information in dark and light areas
in your image do not 100% match the midtones, so there is still some color shifting in those parts.

Anyway since this method does only need a handfull of steps, I do think its worth a share.
If someone can expand this technique I'M glad to hear about it.

n1 Load your image in Affinity


n2 Launch Frequency Seperation.


n3  In the dialog try to bring as much detail as possible in the high frequency layer, but avoid
any lightning influence in it,.This step is quite important, and you might try different values. 

This is where Affinity Photo really shines since it gives you a zoomeable preview of both outputs. Wich takes away the guesswork
you need to use in Photoshops own manual frequency seperation.


n4 Copy the Low Frequency Layer (Strg +J) and invert it (Strg +I)


n5 Set the blending mode to overlay. Now you will loose some color information, if you do not want this, desaturate your overlay layer.
But be aware that you might have problems blending the seam of those layers together. Since there might be also some
color shift.... Another blend mode to consider is multiply with a 50% opacity. Wich does make your image a lot darker, but is more
forgiving on the color information itself.


n6 Merge the layers down. Use Filters->Distort->Affine.. to check and paintout the seams


As said its not the holy grail, but just another method to get rid of unwanted lightning information.
Very dark and light areas might still be included in the final result
(But as said in most cases the color information in that area is also quite low in quality)

For the above example I had to run the process twice and blended the second result with a 50% blend to get it seamless. The final result
(wich includes a pretty sloppy stamp brush job) can be downloaded here