Three pillars of exposure: Aperture

1. Aperture can be closely related to the eye. For the reason of the eyes focus on specific objects. And in this case it is thew cats head as you can clearly see here.

2. The smaller the aperture size, the higher the aperture number.
3. Aperture Depth of Field is important to remember. So if the aperture is small then the area of focus is large, if the aperture is larger the smaller the focus are of a photo.