Test setup, 10D + 135/2L + 550EX flash.
A strip of paper is placed over the middle of the lens...

f/2. Note weird bokeh in background highlight by my left ear.

f/2.8. Background highlight is smaller, strip missing is same size.

f/4. Background highlight is almost gone, and so is contrast.

f/5.6. Background highlight is almost gone, as is contrast.

f.8. Background highlight? What highlight?

f/11. We are very close to complete occlusion here.

f/16. The paper strip is now completely occluding the shot.

f/22. Same deal.

As you can see, as aperture is decreased, so is the area of the front element that is actually used in the exposure. The paper strip completely covers the portion of the lens that is involved in exposure at f/16 or smaller apertures. Larger apertures use more of the area of the front element of the lens; the larger the aperture used, the less significant the effect of the paper strip is on the final image.