You have likely seen photos on the internet which are well-exposed at the center, but the brightness starts reducing towards the edges. If you are not familiar with the name of this effect, it is called an edge vignette or image vignetting.
Interestingly, a vignette is an effect which occurs naturally but can also be achieved intentionally during the post-processing stage. Not everyone likes the vignetting effect in their photos, for some, it is a useful technique and for others, it is... more