15s is the longest exposure the S95 is capable of.
Think of ISO, Aperture, and Shutter Speed as an equation.
0 + 0 + 0 = 0 (hypothetical perfect exposure)
If you move one value up or down, another has to balance it out.
(-1) + 0 + 1 = 0
Since you can control all 3 values in 1/3 increments on the S95, an example would be.
ISO 400 + F4.0 + 1/1000 SS = Perfect Exposure
ISO 320 + F3.5 + 1/1000 SS = ^ (the above) Will result in slightly smaller depth of field since aperture was increased as well as less noise due to lower ISO.
ISO 400 + F4.5 + 1/800 SS = ^ Will result in slightly larger depth of field as well as less ability to freeze motion due to slower shutter speed.
That's how I explain it now to people who are new.