Balance in photography is when an image has subject areas that look balanced, or even, throughout the composition.
There are 2 different ways to do balance.
1) Formal Balance, or Symmetrical
Formal (or Symmetrical) Balance is achieved when both sides of the image hold equal weight.
As you can see by the two examples below. Each photograph is equally balanced if split down the middle, yet one is literally symmetrical and the other uses different elements of the composition to appear symmetrical.
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2) Informal Balance, or Asymmetrical
This type of balance is a bit more difficult to achieve. Many professional photographers prefer asymmetrical balance due to the complexity of the image. When an image is symmetrically balanced, that symmetry is initially obvious to the viewer. However, when an image is asymmetrically balanced, the viewer has to spend a little more time looking at the image in order to realize that fact.
Instead of mirror images or an equal weight on each side of the photograph, the image is balanced by the creative use of size, tone, and form of the subjects within the composition. Each side of the photograph, in turn, becomes equally weighted regardless of their differences.