A group of men in suits and fedoras tour Ed Mitchell’s potato crop in Tomales; a young Boyd Stewart plants oats from atop his John Deere tractor in Nicasio; and, on a tract of land where the San Geronimo Valley Golf Course now stands, horses pull a mower through a sun-drenched field of vetch. These scenes and many more were on display at this year’s Marin County Fair as part of the photography exhibit, “Marin’s…