aperture

F2.8 aperture

F16 aperture 

1. The human eye.

2. The smaller the aperture (ex: F16), the less light let into the photo, therefore making the depth of field larger. And the larger the aperture (ex: F2.8), the more light let into the photo, which makes the depth of field smaller.

3. The larger the aperture is (ex: F2.8), 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 (ex: F16)  the more depth of field you will have, which blurs less of the background and makes the foreground bigger.