Merlin (Yahoo!, FZ10 group, 15 June 2004) writes...
I find the best pictures result when the aperture is around 5-6. For this reason, I shoot in A{perture} mode and generally try to stay in that range, provided I get an adequate shutter speed. This of course does not apply if I am after the specific effects that a 2.8 or 8.0 offer...I must also point out that although we all use the same camera, we *do not* all use the SAME camera. In other words, setting my camera on High sharpness {or saturation, or contrast} may result in the same amount of sharpening as someone elses FZ set at normal.Doug Wilcox (alternate photo-gallery) (Yahoo!, FZ10 group, 15 June 2004 and 16 June) adds...
When a lens is wide open (FZ10 f2.8) the lens is never as sharp and the depth of field is very limited. The sharpest part of the lens is located in the middle of the field so stopping down a bit will make a bit sharper image.
On the other end of the f-stop range (FZ10 F8) you are introducing some funky light refraction problems. Sometimes these can be used for effect and of course the smaller lens opening makes for a much wider depth of field but the picture is not as sharp as somewhere in the mid range of f-stops. I have actually seen test reports that name a specific f stop as the sharpest for a particular lens but in general somewhere in the mid range is best.
...no lens is sharpest at it's widest aperture. This is not only a factor of Depth of Field but also because more of the lens element is coming into play at wide apertures. When you stop down an lens so that the lens area used is closer to the focal point of the lens it is sharper. The lack of sharpness at smaller apertures (f8) is more a factor of the way light bends around a small lens opening, (refraction).