Cropping for better composition
Cropping is an editing feature that allows you to trim off parts of the image that you don't like—it essentially provides an opportunity to recompose the shot after it has been taken.
However, one vital point to remember is that cropping discards pixels and therefore lowers file resolution. I love photographing birds, but unfortunately, they always tend to be too far away, even with a 400mm telephoto lens, so I have to crop the edges off the file to make the subject appear larger. If I crop 50% from a photo, it then looks as if I have shot the subject with an 800mm lens, not my regular 400mm lens. Cropping has saved me a lot of money so that I don't have to buy an even more powerful (and thus very expensive) lens. But the inevitable compromise is that, in that example, I have lost half the pixels in the file—no problem if I only ever post the image online, but it will be a restriction if I hope to have it printed large.
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