Vignetting
The process of making the image's corners appear darker. Burning in the corners of an image digitally in Photoshop is one way to get this look, which is typically done on purpose to draw attention to a subject that is located in the middle of the picture. Toy cameras, such as the Holga, are also known to produce photographs with this effect rather frequently. Unintentional vignetting is typically caused by lens fall-off, which is something that may be remedied with the help of post-processing software.
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