My big lens choice for my Colorado trip was the Canon EF 200-400mm f/4 L IS USM Lens with the built-in 1.4x extender. The decision to bring this lens was not a difficult one. I was going to be primarily shooting landscape with access to certain views limited to very long distances. I also planned to photograph wildlife in a range of sizes when the opportunity presented itself. For both situations, the zoom focal length range was more important that a (potentially) 1 stop wider aperture for this lens choice.
I came across this large, fresh-out-of-the-wallow, bull elk trashing a thick clump of small trees with its antlers. After shooting this activity for about 20 minutes from a bad position (from-the-rear was the only angle available to me), I decided to move on. I didn't have much time in this park and still had a long distance to cover.
I was back at the SUV with the lens and tripod torn down/compacted for transport in a Think Tank Photo Airport Accelerator backpack when I saw the bull finally leave the thicket (I think one of Murphy's Laws covers this situation).
I rapidly re-deployed the setup and worked my way to the opposite edge of the clearing that the bull had entered. If I had a 400mm lens prime lens, I would have needed to move back into the woods, making a clear shot far more difficult. A 300mm prime lens would have framed the scene wider than I wanted. With the zoom, a quick adjustment to 350mm was all that was needed.
My preference is to shoot wild animals at their level (a below-level vantage point also works well sometimes), so I setup the Gitzo GT3542LS Tripod in its fully retracted position. Getting a clean background was not going to happen, but I like the trees being present in this case. I did make sure that the bull's head was framed between trees. I adjusted my position to get a just-slightly-forward of a direct side perspective with the head framed between the trees. In this position, a large number of focus points land on the desired plane of sharp focus that includes the all-important eyes. When the bull bugled, I was ready.
You can't tell in a reduced-size image, but even with a wide open f/4 aperture being used, this image is razor sharp even when viewed at 100%. This encounter with the large 7x7 bull elk was another confirmation that the 200-400 L is, in very many cases, the ultimate wildlife lens.