I tend to gravitate to exposure times of more than 8 seconds and less than 20 seconds and avoid the lowest f stop and f stops over (focal length/4). The first is for control. Timer error and time for manipulation are factors for very short exposure times. Very long times seem less effective as well. As far as f-stop - It is my understanding that the threshold for diffraction in the iris of a lens is about (focal length/4). That would be a good max of f22 for my 150mm lens and f16 for my 50mm lens. I have read posts here that indicate some of you are using very short times. Is there a benefit? Are you just using the "sweet spot" on your lens? What about times over 30 sec. I hate these long ones. My old timer really starts to stick over 30 seconds. What have you observed??