>I just noticed that when photographing objects at great distance (distant mountains, etc.), the autofocus won't focus >at a very wide angle (from about 24mm to the minimum 18mm). >This problem is most severe when the camera is held >horizontally (landscape). When the camera is held vertically (portrait), it usually focuses. snipped >Sometimes the lens makes a bit of noise but doesn't "beep" to indicate focus, and sometimes the lens >simply does nothing. Congratulations You are accurately describing an in camera AF limitation mentioned on page 70 of your D300 instruction book - and at http://nikoneurope-en.custhelp.com/app/answers/detail/a_id/4585 Your camera is showing common behavior for what you are trying to use it for. In camera AF has difficulty in reading either low contrast (example 1 in the Nikon guidance) or very fine detail (example 6). Distant mountains are often low contrast and wider angles make the detail smaller - which is why you get the majority of issues with distant mountains and wider than 24mm. The outer AF points on the D300 are single line reading top to bottom of the viewfinder - and are more likely to find good for focusing detail when the camera is held vertically. Digressing the "extra AF difficulties" with wider angles are mentioned in lens instruction for DX lenses going to 18mm or wider. The common work around when AF cannot detect the subject is to switch to manual focus and for distant mountains to set the focus ring a little closer than infinity distance.
Photography is a bit like archery. A technically better camera, lens or arrow may not hit the target as often as it could if the photographer or archer does not practice enough.