a better way to say it, the Ixus series cameras have a fixed F/2.8 aperture. this is because with such small cameras, you have to sacrafice somethings, in this case its an adjustable aperture diaphram. the nd filter clicks on in bright lighting conditions to simulate an F/8.0 aperture, so that you can capture pictures in bright lightning conditions. Thats what i hated about my SD1000 i used to have, you just can't get creative and use depth of field in shots, you can however simulate it in photoshop if you know how.
now i have an A590IS, its got an aperture (finally i have a camera with one!!), and also manual focus (old camera lacked that) so its alot better, but no firmware builds available for it yet, but they're in development ;-)
