On our November 2011 jaunt to Bosque del Apache, I convinced Jeff Heidel to go along. He was concerned that he didn't have a long enough telephoto lens. At Bosque, the big lenses are sometimes beneficial but not exactly necessary. On a crop body, a 70-200/2.8 with a 2X will usually be more than enough. First day of shooting I noticed Jeff seemed to be struggling. Only noticed this because he was fairly close to me and I could hear him mumbling and grumbling about his Nikon lens. His images were fine but the lens and body were searching for focus way too long. I know his equipment is at least as good as mine so I figured there had to be something in his setup. Quick look and I knew. Almost all the top brand telephoto lenses, especially in the pro series from Canon and Nikon have focus range settings.
The range switch on my Canon 100-400 has two settings; 1.8 Meters - Infinity and 6.5 Meters - Infinity. Photographing the birds at Bosque, we do get close but rarely inside 6.5 meters, about 20 feet. Jeff's lens was set for the full range. I asked him to switch it to the long limit setting and try that. From right next to him I could hear the grumbling change to "Oh WOW!" "Awesome!" "Great!"
For the rest of the trip he was turning out exceptional images with no complaints! It helps to know this and be told this but it is tough figuring it out on your own. I learned the hard way with my 100-400 back in the film days when I seemed to struggle with searching focus on that lens.