Why does a wide aperture blur?

We all know that a wide aperture blurs the background, but have you ever considered why? It’s all to do with the way the light travels into the camera.

If you have the lens set to its widest aperture setting (f/1.4 or f/2.8, for example), this means the aperture ring is opened fully, and the lens can take in much more light at a much wider angle. Therefore the light has less distance to travel before it becomes out of focus.

When you have a narrow aperture setting (like f/22,

although even at f/11 the scene is pretty sharp), the light has to travel further before becoming out of focus, therefore more of the scene stays sharp.

When we refer to the depth of field, we’re talking about the distance between the closest part of the image that’s in focus to the furthest part. The depth of field is not only affected by what aperture setting you select, but the depth proximity of the subjects. If your subjects are all very close together, they will be less blurred; if they’re further apart, the blur is greater.