A friend of mine is a very good portrait photographer. You might want to check out her blog (Click here) One of the key elements of much of her work is the use of negative space. According to Wikipedia, "negative space, in art, is the space around and between the subject(s) of an image". Today's image uses negative space to frame the tropical flowers. It was taken in overcast skies. The appearance is such that the impression is that a shaft of light magically shown down on the flower and the rest of the area was under the clouds. Almost, the shaft of light comes from an off camera flash set to TTL exposure determination. The camera itself is set at F5.6 at 1/250th of a second. A reasonably "standard" setting for a Nikon DSLR. Because of the distance to the background elements, F5.6 works fine to produce the "bokeh" of the scene. The reason for the crop being the way it is is due to the light spots at are so soft that they almost look like clouds hanging over the blossom. It also gives the flower some space to "grow" into. Unless you're intent is to do a dramatic crop, cutting into the subject on several sides, the subject needs a little "breathing room". By "dramatic crop" I'm talking about cutting into the subject to create emphasis on a particular piece of the subject. When "the guys" were young we used to send out holiday cards to the relatives. The first was of our older son and consisted of his eyes only. You couldn't see his nose or hairline. Just his eyes. That was a dramatic crop and caught the attention of everyone receiving a card. From then on people looked forward to each year's offering. To find out more about "negative sapce", hit the "read more".
Negative space doesn't have to be empty space. It's function is to concentrate the viewer's eye on the main subject. It shouldn't compete with the subject and the most common form of negative space is formless. You probably can't tack up a calico cloth, keep it in reasonably sharp focus and call it negative space. On the other hand, a totally out of focus calico cloth, where the circles of confusion are so large as to blend the colors into a soft, mottled emptiness might work fine. Dropping the exposure on such a backdrop probably wouldn't hurt.
The use of negative space comes down to control. You cannot let the negative overpower the "positive". The "subject" has to stand out. No one should have any doubt where to look when viewing an image with a large amount to negative space. When used properly, negative space can be a powerful picture element.