Many years ago I took a photography class with the great fine-art photographer Michael Kenna, whose exquisite black-and-white photographs of landscapes, especially at night, are still considered classics. Every week, before class started, the students would hang their photographs (or “work” as we called them) on the wall. Kenna walked into class holding two L-shaped pieces of cardboard and immediately went to work on the photographs. He used the L's to crop each photograph in a variety of ways until he coaxed a much stronger image from the photograph.
Cropping, the removal of unwanted areas of an image, is probably the most basic photographic manipulation. In film days it was done in the darkroom or even by trimming the photograph itself. Today it is a basic function of photo-editing software.
I must admit, though, I took Kenna's cropping lessons with a pinch of salt. That's because I was grounded in traditional photographic discipline which strongly discouraged cropping. Following the French great Henri Cartier-Bresson, many black-and-white photographers eschewed cropping. To prove this they printed their negatives to include a thin edge around the image corresponding to negative rebate. So their photographic prints contained a thin black border around the image. If you look carefully at any of Cartier-Bresson's images, you will see black borders, proving it was an uncropped image.
I should explain the reason for this belief. Photographers strongly felt that the image should be “composed” or arranged within the viewfinder. When taking the picture, if you thought “Oh, I can fix that photo later with cropping,” you would not strive for the strongest possible image and become a lazy photographer.
I certainly believe this for most types of photographic subjects, such as people, landscapes, buildings, streets, etc and try to compose in the viewfinder without having to resort to cropping.