Sounds like a great trip ... I love driving the southwest.
When my wife and I moved to LA from NYC we tried to find and drive as many sections of old Route 66 as we could.
Never been to Carlsbad Caverns. But from what I've seen online, the caverns are lit up with some dramatic lighting.
If I were planning on taking pics there I would certainly be thinking about bringing a fast wide-angle lens. From the online images, it looks like the caverns have areas where they are big and massive and other areas which are more closed in and tight. When you are inside them, I'm pretty sure you're going to want a fairly wide-angle lens to capture the scope of the place.
Some areas in the photos I've seen look very well lit, while many others are lit with colored lights with lots of shadows.
I have a 17-55mm 2.8 lens for my Canon 7D. I would certainly bring something like that.
If you have a prime wide-angle that's really fast, I'd definitely think about bringing that along.
Personally, I would also bring along my Lumix DMC-LX5 which has a super fast Leica lens. I'd use that to shoot 16:9 format shots in the widest setting for a panorama look without needing to photo stitch. That camera creates wonderfully saturated colors ... perfect for something like these caverns. I have to say, so many times I end up preferring what I've shot with this "pocket" camera to what I've shot with my big and heavy 7D.
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