For the person who paints or draws, art is about addition. The artist starts with a blank canvas or piece of paper, and then adds whatever elements are needed for the composition. For example, if the artist wants to paint a landscape, he might add a tree, a cloud, some mountains, or whatever else he wants to appear in his painting.
Photography, on the other hand, is about subtraction. The photographer starts with everything that is in the viewfinder, and by various methods, subtracts what is not wanted in the composition. That beer can in the foreground of my landscape? I have to physically remove it, or change my camera position or zoom my lens slightly so it doesn't show. If I am really lazy, I can remove it later with software. Since the camera gives equal emphasis to everything in the viewfinder, the photographer has to decide what doesn't belong, or what distracts from the main subject, and subtract it. The failure to subtract non-essential or distracting elements from a composition is often what separates good photos from mediocre work.
One Left, Waynesville, NC - 2013
(Click on photo to enlarge.)
On the photo above, I wanted to isolate a single red leaf that was on a bush that still had hundreds of leaves, many still green. I did this by zooming in close to eliminate the other leaves, and then going with a square crop on the computer to further eliminate any other distracting elements. I titled it "One Left" because it looks like the last leaf left on a branch, when actually it's one of several left. The painter or photographer can create their own reality. A painter would have added only one leaf to his painting for this objective; as a photographer, I had to subtract everything else to get the composition I wanted. Math, you just can't seem to get away from it!