Re: Lenses for Yellowstone
2
You need to give serious thought to how you use your time in the morning. Many geothermal features, particularly Norris Geyser Basin, look entirely different in cooler morning air than they do once it warms up. Features that you won’t even notice in warmer air jump out in cooler air. It can make for amazing photography.
For example
Early morning at Mammoth Hot Springs - you lose that misty glow as things warm up.
Mid morning at Norris - most of those thermal columns disappear as the day goes on
It is often said that you need longer lenses for wildlife in Yellowstone. That is IMHO only somewhat true. For bears and wolves you need crazy long lenses most of the time (think camera on spotting scope) … unless you get lucky and happen upon one at close range. Then you have choices. However for buffalo and elk you often don’t need long lenses at all. Usually the issue is not getting close enough to get inside their personal space (at the cost of your own continued health).
Take these two shots from a series of a young grizzly bear at dusk under cloud cover….
Typical view of Yellowstone grizzly. Fairly distant. Usually more distant than this.
Young grizzly at close range after it changed direction causing everyone to scramble back to car
Here is a fairly representative shot of an elk at Mammoth Hot Springs. This was one of two dominant bulls that used to run the area. Sadly both have passed away. Both were very ornery in the Fall, and would charge anything that moved. Including cars, cement trucks, and unwary people walking out of brunch.

My ultimate point is that I would first and foremost scale my lens expectations, and time usage, for geothermal areas. Wildlife is going to do whatever it does. Most of the time they will be too far away for any lens to matter. Most of the rest of the time almost anything you have will do. For that small remainder where you need a long lens - it is really hit or miss. Of the 5 times I’ve been to Yellowstone, only twice have I seen wolves. Same with grizzlies. Of those two trips where I saw grizzlies, only one were the grizzlies closer than spotting scope range. Of the two trips I saw wolves, one trip had one wolf sighting and one trip had two sightings. Of those three wolf encounters, one was spotting scope range, one was with a Nikon 70-200 through some trees, and one was on a m43 with a 100-300 - and ultimately they closed on me (I was hiking alone and I was lucky they were only curious) so that I was on the lower end of that lens’ range.
Enjoy the trip, get ready to want to go back, and don’t stress about the lenses. Use your morning time wisely.