[JohnC]

It is quite easy to be. The lens on the Canon S2 is a 6mm to 72mm zoom. It is a variable aperture. At 6mm it is f/2.7, at 72mm it is f/3.5. If it was a constant 2.7 it would have to be much larger, heavier, and more expensive. 2.7 isn't all that big an aperture for such a short lens.

The S2 can probably not stop down past F/8 due to the short focal length, so the range of full f/stops is 2.7 (3.5), 4, 5.6, and 8. And also because of the short focal lengths it will be practically impossible to do any selective focusing, too much depth of field. This might also cover any small focusing inconsistencies.

And the lens only has to project an image circle to cover a tiny sensor (about 4mm x 5.5mm) instead of 24x46mm. And the lens is built specific to the body, no worry about a mirror being in the way, so it can be as close to the sensor as it needs to be.

My guess is that someone could manufacture an 18-200 2.8 constant aperture zoom lens. But I don't know if anyone would want to deal with the size, weight, and cost.

John