When I teach a class on photography many people struggle to grasp the idea of Depth of Field. What is it and how can it effect their photography.
So lets try to explain what is it?
Depth of field is the distance between the closest and farthest objects in a photograph that appears to be sharp or is deemed to be acceptably sharp. Your camera can only focus on a single point. So everything in front and behind that point is not perfectly sharp but we perceive it as sharp.
Using a small aperture f22 will give you a large DoF while a large aperture such as f2.8 will give you a small DoF.
See the examples below.
As you can see, as the aperture goes from f2.8 to f32 the DoF increases to show the background clearer and clearer until the whole image is acceptably sharp.
Now looking at the images above you can see for yourself that as the aperture decreases (numbers get higher) sharpness improves.