If the elements of art are the building blocks of art, then the principles of design are HOW those building blocks (elements) are put together in a composition. Or in another analogy, if the elements of art are the ingredients of art, principles of design are the recipes that describe how to combine them!
The principles of design include: rhythm, repetition, unity, contrast, balance, and emphasis.
Rhythm is the repetition or alternation of elements, often with defined intervals between them.
Rhythm can create a sense of MOVEMENT in your photograph, as your eye will tend to follow the repeated elements throughout your composition.
Pattern and repetition refers to repeating visual elements such as line, color, shape, texture or value in an image. This tends to unify the total effect of a work of art as well as create RHYTHM.
Repetition can take the form of an exact duplication (pattern), a near duplication or duplication with variety.
Unity occurs when all of the elements combine to make a balanced, harmonious, complete WHOLE. Or to put it simply, when the elements "go together."
Unity can be created by:
Simplicity (only having a limited type of lines, shapes, colors, etc. instead of lots of variety)
Repetition (repeating similar elements)
Proximity (placing similar elements together)
For example, in the image below there is unity created by the simplicity (all the boats are the same shape and color), repetition (the boat is repeated several times), and proximity (all the boats come together and are in close proximity to one another- not spread apart).
Contrast refers to placing two opposite elements together. This most often refers to a contrast in VALUES (very light areas next to very dark areas, like in the image of the lion below). But contrast can refer to any opposing elements- such as a contrast in different textures, colors, shapes, etc.
Balance describes the way subject matter is placed in a photograph. Balance can be symmetrical (elements are the same on both sides) or asymmetrical (where elements are placed unevenly, but still work together to produce harmony overall).
This "harmony" is achieved by balancing the VISUAL WEIGHT. Visual weight describes how much something in an image "pulls" your eye to look at it. Imagine that you have an almost entirely white image with a small black dot in it. That black dot will pull your eye immediately- it carries a lot of VISUAL WEIGHT.
Balance is a way of evenly distributing visual weight. This way, no single area of the image draws our eye so much that we get "stuck" there. Instead, our eye is free to roam around the image and take it all in. A balanced image feels pleasing to the eye, not lop-sided in any way!
SYMMETRICAL BALANCE:
If you were to fold your photo in half, both sides would be about the same.
ASYMMETRICAL BALANCE:
If you were to fold your photo in half, both sides would be different.
Emphasis is highlighting or drawing your attention to some aspect of a scene or subject.
There are several ways to create emphasis:
Framing
Placement of subject (governed by rule of thirds)
Selective focus to simplify the background (shallow depth of field)
Drawing the viewer's attention to a certain spot using leading lines.
Contrast (ie: one green apple is emphasized in a pile of all red apples)