Excellent article. It helped me to understand masks a little bit better.
I have just one question: Why don’t I see a difference when I use the slider in most pictures? Only the slider for the river picture works. For all others I do not see any difference?
After a reload it worked as expected. So now I learn even a little more 
The whole writeup is a bit weird.
I find many of the corrections are overdone, eg the face of the Lincoln statue and the last forest image.
In the lighthouse image, there is a person (wearing blue) who disappears after the edit. The exposure module is indeed powerful if it can do that.
Yes, I over-exaggerated the effect to make it easier to see. I wouldn’t brighten Lincoln’s face that much when normally processing that photo.
In the lighthouse image, there is a person (wearing blue) who disappears after the edit. The exposure module is indeed powerful if it can do that.
Good catch! I removed that person with the retouch module. I thought about including that step, but it was already getting quite long and using retouch on that image wasn’t relevant to the effect I was trying to demonstrate (using exposure to create a vignette).
The missing person was the first thing I noticed when looking at the comparison and it totally distracted me from seeing any vignette effect 
Fair enough, I re-exported the “before” picture of the lighthouse after the retouch module so that should no longer be a distraction 