What's focal length and aperture got to do with Depth of Field?

This photo shoot started off with the intention of showing the relationship between focal length of the lens and the aperture on the depth of field. The subject of this shoot, leader-class Optimus Prime was patient enough to stay around in more or less the same pose through the shoot without tipping over even once. These shots involved moving Prime away from the camera rather than moving the camera away from prime, so there will be some discrepancy over the backdrop of the photos through the 3 different focal lengths.

The focus was on his head, with arms extend back and forth to show how the depth of field keeps increasing as the aperture keeps increasing in number (or decreasing in size)

First in the series, is Prime at 18mm on the 18-55mm lens:

*Note how the vignette in the first pic at f3.5 disappears at f5.6

Focal length = 18mm (28mm in 35mm) at f3.5

Focal length = 18mm (28mm in 35mm) at f5.6

Focal length = 18mm (28mm in 35mm) at f8

Focal length = 18mm (28mm in 35mm) at f22

Prime at 28mm on the 18-55mm lens

Focal length = 28mm (40mm in 35mm) at f4

Focal length = 28mm (40mm in 35mm) at f5.6

Focal length = 28mm (40mm in 35mm) at f8

Focal length = 28mm (40mm in 35mm) at f11

Focal length = 28mm (40mm in 35mm) at f22

Prime at 55 mm on the 18-55mm lens

Focal length = 55mm (88mm in 35mm) at f5.6

Focal length = 55mm (88mm in 35mm) at f8

Focal length = 55mm (88mm in 35mm) at f22

Focal length = 55mm (88mm in 35mm) at f32

Focal length = 55mm (88mm in 35mm) at f38

If you've seen all these photos, you've probably also noticed how much larger Prime's first appears as compared to his head in the wide angle shots as compared to the telephoto shots. This is because at wide angles, objects closer to the lens will appear larger than objects even a short distance away from the lens. In this case, his fist and his head.

And in other news, thanks to this shoot, I've found something (a fibre) is probably stuck to my lens. Time to clean it out I guess...