Hi there, Tamarack,
Welcome to the Q&A Board.
I think the "sweet spot" reallly depends on the type of photo you're taking and what depth of field you're trying to achieve. In general, many opine that to get the sharpest results with the S2 use an aperture setting in the F4.5 to F5.6 range.
if you were to name the top five camera-related factors that lead to sharp, clear luminous photos with the S2, what would they be?
Photography, as it's essence, is about light. If you don't have sufficient light, it doesn't matter what your settings are imho. It's difficult to limit the top factors to five, but here are the S2 settings I find give me consistently sharp photos:
AutoFocus mode: Single
Image Stabilization: Shoot Only
ISO: 50
AE/AF Lock: knowing when and where to lock focus and exposure by using the two step shutter release button correctly (Check the S2 manual to learn how to lock either Exposure or Focus manually).
Focal length: the best length varies with the distance the lens is from the subject but I get the sharpest images when NOT shooting at full tele or full wide angle. For example, if I fully zoom in on a subject then move the lens back slightly wider, the images often are sharper.
You may find this section of my S2 blog helpful, particularly the area about focusing:
http://www.digicamhelp.com/canon-s2-blog/canon-s2-focusing.htmGive them a try and let us know your results.
Related reading: Depth of Fieldhttp://www.digicamhelp.com/taking-pictures/depth-of-field.htm