Well, you can go into detail about the “rule of thirds” (more about this another time) you can talk about the lighting, the colour, the tone, the expression and guess what. All of these would be correct. What makes a photo great is the combination of all of the above techniques and more. A great photo is about what speaks to you personally. Great photos will show emotion, will exude some sort of passion, will cause you to think, will make you laugh, or even cry. A defining image is not simply about a topic or a person, its about what it says about those things. You will know when a photo is great…its the one that speaks to you without verbally saying a thing…
