Negative space, by definition, is the empty space around the subject or focus of the photograph. If the edges of the photo frame the contents within, and the subject is the focal point, it's possible that just about every photo, even macro's and tight portrait shots can have negative space.
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To really illustrate how powerful negative space can be though, the photo above shows how powerful nothing in the frame can be. Don't read too deeply into that though, sometimes the negative space in a photo's composition is just as powerful and visually appealing as the focused subject. Take this photo for example.
Here, the focus could actually be the vertically striped wall that is the negative space.
Since I shared recently how I improved my photography in one night by cropping photos in post production, it's worth nothing that to achieve a nice negative space in your composition doesn't mean shooting super wide angle lenses or stuffing the subject into the bottom or lower corner of the frame. Perhaps one of the more fundamental aspects of photography I learned early on was to fill the frame. Here's a great example of filling the frame on a tightly cropped photo where the negative space really is the star.
Framed exceptionally well, this composition exemplifies how important and dramatic negative space can be in a photograph.
Give negative space photography a try, keeping in mind that the subject matter is still key, but look at composing slightly different with a focus on less, and you'll be happily surprised with more.