A diaphragm iris is used to control the amount of light which passes through a lens to reach to the focal plane (sensor). The hole the diaphragm iris creates is known as aperture, the smaller the hole (higher number) the more depth of field you get, aperture size can effect image sharpness too. the diaphragm iris is a mechanical; structure that physically changes size to control the light passing through it.
Most modern Canon digital compact cameras have a neutral density filter inside the lens to control the amount of light reaching the focal plane, because it is just a filter it does not alter depth of field or image sharpness. typically when it the filter is not in the path of light the amount of light reaching the focal plane is equivalent to f2.8, when the filter is in the path of the light f8.0.
Looking at camera specification lists they don't indicate if the camera is using a neutral density filter or not.
Does anyone know which canon compact cameras have a neutral density filter and which have a diaphragm iris?
Canon Ixus 105 - Neutral Density Filter
Canon Ixus 200 - Diaphragm Iris
Canon Ixus 310 HS - Diaphragm Iris
Canon Ixus 860 - Neutral Density Filter