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[conropl]

There are only a few instances that I would shoot at f/32, and if I did I would expect the shots to be soft. There are diffraction problems at small apertures. Usually f/8-f/11 are ideal ranges to get good depth of field (DOF) and sharpness for most lenses. There are times when I want more DOF because I have something interesting that is close in the foreground, but my main subject is at infinity... in this case I would push to f/18. However, much beyond that and the shot starts get noticeably soft (f/22 to me starts getting unacceptable and is a little better or worse depending on the lens). The exception may be macro where you may want to push it to get more DOF, but you would be using an exceptional lens that is better designed to go to f/22 and it would need to be on a tripod (and it is still not as sharp).

Also as a general rule, shooting at wide open apertures does not usually provide the sharpest images either. For most lenses, shooting one full stop above the maximum aperture would increase the sharpness noticeably. I am not sure about your lens... it is a very good lens and maybe pretty good wide open. However, there needs to be a balance approach here as well. That is, the more open your aperture, the less the DOF is, but that is what separates your subject from the clutter of the back ground which can make for a more pleasing shot many times.

Steve is also correct, hand holding at 1/15 sec exposure is getting dicey. Can it be done... yes, but your keeper rate is going to be miserable and you won't know it until you are looking on your computer (that little LCD screen lies). At those exposures, you should be on a tripod. IS is good, but it is not the cure all, and it does not take care of motion blur. For example, your tree would be very soft if there was any kind of light breeze at all. IS works to help take out your movement not the environmental factors.

As for limiting the top end of the ISO: Limit the top end of your ISO... high ISO = Noise, keep it as low as you can to get the shot you want with the exposure optimized to remove any motion blur and the aperture optimized for DOF concerns. To tell the truth, I never use auto-ISO, I set it myself so I can get the best picture. There are exceptions such as sport shooting where you want to maintain a certain shutter speed in changing light conditions outside. Your comment: "... just trust the camera to do its job". I do not trust the camera to do its job... I never use "P" mode (contrary to a recent youtube video posted here, it does not stand for professional). You need to be in control of your exposure as much as you feel comfortable, and as the experience level goes up, then you start taking more control and your shots will start coming out more like what you have pictured in your head.

Sorry for getting long winded