what happens is that in digicams, the physical focal length of the lenses is very very short already, in the region of 5mm. as aperture is expressed as a division of the focal length, an ƒ/8 on a digicam would be the same as ƒ/80 on a 50mm 35mm format lens. when it comes to such small physical number, the actual absolute number becomes important, since a 5mm lens at ƒ/8 gives you an aperture of only 0.625mm which is VERY SMALL and hard to engineer properly. also at that kind of microscopic size, diffraction becomes an issue.
