Load the photo in Adobe Lightroom
As you can see, vignetting or darkness of the corners can be very disturbing, especially in pictures where there are homogenous surfaces in the corners. The sky is a good example: this problem is much more apparent there.
The picture above — although not much of a good photo— has the same defect. Good news is that this is no trouble for Lightroom RAW converter.
Only 2 sliders
What is more important, it demands little effort from the Lightroom control user too. You simply need to find Vignettes section among the right-side control rows of Develop. It will be around the bottom of the list.
Here you can manage darkness of the corners with the help of two sliders in the Lens Correction window. Amount makes darkness disappear or it increases it instead, depending on the direction of the slider. Dragging the slider to the right makes the corners lighter, while the left direction makes it darker. Midpoint modifies the size of the vignetted area. If you set it to a low value, the Amount slider will influence the corners only, but if you set it high, it will affect a bigger and bigger area going inwards, including the corners and edges.
Personally, I like to start with Amount. I set it to either the lowest or the highest sum, that is, I turn the corners very dark or very light. This makes it easier for us to set the second slider and the size of the darkness to be removed because the area to be changed is more visible. Having set the Midpont as explained above, I increase or decrease the Amount value until the corners become approximately as light as the central part.
You can check the difference between the original and the modified state with the help of the switcher in the top left corner of the palette.
The Vignettes palette contains a Post-Crop section as well, which allows for the post-editing of the darkness in the corners. However, this tool facilitates post effects, for example if our aim was to make the corners rather dark or we intend to create a simple frame by vignetting.
Amount and Midpoint sliders can be found here also, but apart from that, we can define the shape of vignetting with Roundness. A low value will result a more angular darkening, while a high value will make it completely round-shaped. Feather (softening) slider sets the length or direction. Drag it to the left to get a frame-like transition, to the right if you would like a softer transition.
A simple frame
Check what happens if you drag Feather slider to the left completely. The result is a thin black frame with a sharp contour and somewhat rounded at the corners. With Lightroom you can create a simple black or white frame, or you can even make it a bit transparent if you like.