i have been reading several articles in Popular Photography & Outdoor Photographer Magazines concerning the proper f-stop for landscape photo compositions. i think they really complicate matters with fancy charts that require math skills i apparently do not have. i think it's much more simple than it's made out to be. For reference, i will say that i exclusively use a Nikon D200 and most often an 18mm - 135mm lens for landscape images. Simple rule i use: The further out the lens is extended, the lower the f-stop should be.
The following 2 images are of Death Valley's iconic Zabriskie Point. These were composed in April '10 and not considered keepers for me as the weather was too flat that day, but they serve this purpose of demonstration well. i do think both shots are crisply focused however. In the first image, the wider-angled one, the lens was all the way back and the image was composed at F-16 @ 18mm. In the second image, i wanted a closer image of Manley Beacon, so i extended the lens out to about 110mm. Both images were composed from the exact same location. When i extended to 110mm, i decreased the F to 11. If i were to extend further to say 135mm, i would have went down to F-8.
When your lens is extended, it increases the surface area upon it to be influenced by wind or tripod vibrations that lead to camera shake and soft images. Speeding up the shutter speed (decreasing the F-stop) is a good way of countering this effect. Through much practice, i have settled on F-16 as the ideal F-stop for wide-angled landscapes. The great thing about digital is that allows one more room for experimentation.