Informal Balance can be defined as Asymmetrical Balance and is not as easily noticed. Informal Balance can be achieved when dissimilar objects balance each other out on different sides of the frame. When successfully achieved, asymmetrical balance is much more interesting to look at than symmetrical balance.
In this photograph, the shoes become smaller as they get further away. Though this would cause the image to appear to be unbalanced there is a house behind the smaller shoes, which balances it out, making it informally balanced.