Aperture

f16

f2.8

1. The human eye.

2. The smaller the aperture, the less light let into the photo, therefore making the depth of field larger. And the larger the aperture , the more light let into the photo, which makes the depth of field smaller.

3. The larger the aperture is, the less depth of field you will have, meaning more of the background will be blurred out and the foreground will be smaller. And the smaller the aperture is  the more depth of field you will have, which blurs less of the background and makes the foreground bigger.