Cropping is used to extract only a portion of an image and discard everything around to either focus the attention on a precise thing or get rid of some disturbing environmental things. For instance you take a general picture of someone and crop it just to keep the face and make a portrait, or you crop a landscape to remove that power line on the right side. It can also be used to give some power to a dull image by changing the composition and making an image compliant with the rule_of_thirds although it wasn’t at start.
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