This was a big trip for me and I did not want to be limited by the gear I was taking. Therefore, I spent a lot of time thinking about and researching my wildlife lens selection for Katmai National Park, Alaska.
Choosing a lens for the first visit to a location must be based on advice given by others and on understanding/visualizing the situations that will be in front of you. As indicated, ultimate image quality was a baseline for my decision making process. The lens focal length was another primary consideration as perspective, framing and background blur are strongly influenced by this choice, affecting the look you get in your images.
Brown bears were my primary subject in this location and a significant interest I had for this trip was to capture frame-filling brown bears in action. For that purpose, I needed a long focal length lens with excellent AF performance. Since the weather conditions could range from sunny to full-on rain, I decided that a wide aperture was also needed (f/4 or wider) for those darker days.
One thing that I knew was that my Canon EF 100-400mm f/4.5-5.6L IS II USM Lens was going to be along. This lens has everything I wanted for a wildlife lens including a size and weight that I could carry for long distances and handhold, but this is an f/5.6 max aperture at the 400mm end.
Which big lens to accompany the 100-400 remained the question. I love my Canon EF 200-400mm f/4L IS USM Lens for wildlife. That I already had this focal length range covered by my 100-400 was not a big decision factor as the 200-400 had the significant benefit of a 1-stop wider aperture. But, that I was uncertain that 400mm was going to be long enough was a bigger factor. The 200-400's built-in extender takes this lens to 560mm with the throw of a switch, but a 1-stop impact on the max aperture yields f/5.6. An f/4 max was my preference. Note that I was not at the popular Brooks Falls where the 200-400 L lens may have been the first choice.
Another option was to rent a Canon EF 500mm f/4L IS II USM Lens. This lens would give me an extra 100mm and still have an f/4 aperture. The 500 f/4 is considerably smaller than the 600 f/4 and easier to pack, carry and use.
In the end, I made the Canon EF 600mm f/4L IS II USM Lens my primary long wildlife lens choice. The longer focal length paired with an f/4 aperture is what was the primary decision maker. It seems that wildlife is never close enough and if it does get too close for 600mm, much harder-to-get headshots and similar become possible.
When setup on location, I had the 600 f/4L on a Wimberley Tripod Head II mounted to a Gitzo GT3542LS Systematic Carbon Fiber Tripod. A second camera with the 100-400 L II mounted was at my side, ready for capturing environmental-type images or for closer subject distances when needed.
Packing the 600 around Alaska required some effort, but I was very happy with my decision. Many of my images would have required cropping (or more cropping) if a shorter focal length was used. And, an f/4 aperture along with the required action-stopping shutter speed meant that ISO 3200 by far the most used as about 50% of the time in the field was under dark skies with light rain. I know, some of you are thinking that 600mm would be about 1/2 as long as preferred to keep an acceptably long distance between yourself and the bears.
The mamma brown bear in this image was snorkeling for salmon. Each time that it would lift its head above the water, it would shake. It didn't take long to figure out this behavior and I began timing a burst of shots as the water flew. A 1,000 lb animal shaking a significant amount of water from its fur is an impressive sight. A window in the cloudy skies gave me enough light to use f/5.6 for this image, gaining a little depth of field to keep more water droplets in sharp focus.