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Hi Pierre...
I'm not sure what the 'brightness' setting is that you are referring to,
but using a wide angle lends near a city is always going to show some gradient due to light pollution.
"stretching" the image in processing by increasing the exposure level, and then also increasing the contrast will make this gradient more apparent...
you could try using something like graxpert to smooth it out but i'm not sure how much it would help
The lens you are using is known for light drop off in the corners, and i think i see a bit of vignetting in the top right corner? This could be a factor as well.
You make also have a bit of stray light hitting the front element - you could try a lens hood?
you may not have seen any cloud, but it sure looks like you had some....
I'd also try stacking more than one image - single images are really hard to get right
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