In this series of photos, you can see how adjusting the f-stop, or aperture, affects depth of field. In the first photo, the door knob is a distracting element. Lowering the f-stop number – or increasing the size of the aperture – reduces the depth of field. That blurs the background, making the door knob less distracting.
All six photos have the correct exposure. But which photo has the creatively correct exposure? I’d argue that it’s the last photo, which blurs the door knob the most.
See another exposure exercise here.