Leading lines are visible lines within the photo. They can be almost anything: shadows, roads, or other objects with visible edges. These lines are very important in different pictures, as they draw attention to the subject you want to photograph.
Of course, you will not find these lines around every photo you want to take, but if there is anything you can use, do so. Note that these lines should always lead into a picture, not out of a picture because they help your viewer’s eye to keep track of the picture.
Making the leading line diagonal will often improve the picture, too.
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