Hi on August 13 at 9.30 am I will have my turn to visit the Lower Antelope Canyon. I own a Sony alpha 7 III and I'm torn between using my Samyang fe 18mm f2.8 fixed focal length or the Sony fe 24-105 F4. Do you have any recommendations? Even on any machine settings, obviously hoping it's a sunny day. Unfortunately I know that the guides tend to be hasty and I would like not to waste my time, also because I think it is not recommended to change the optics in the canyon.
This isn't a great picture. It might help you to picture the conditions. Many of the shots you'll see of Lower Antelope don't have people in them so it's a bit hard to judge what the sizes and distances are. IIRC, portions are deeper and general width varies as well as the amount of light you get.
There are some chambers where you'll stop for a while and the guides will point out particular features, assist with shooting you/your group with your camera. I think it was mentioned to hold back a bit. I sort of did that. It takes a while for a group to move through some of the narrow spots so if you are one of the last folks, you have some time available. That said, it's becoming increasingly popular and busy so they do keep the groups moving.
This one was 1/60s f/6.3 at 17.0mm iso4000 with a Nikon D7200 and 12-24/4 aps-c zoom so 18-36 in ff terms. Glancing at others, some were slower, down to 1/25s and iso 6400. You'll be there a different time of day and year so light could well be different. My exif says 8:11 but I'm pretty sure it was actually mid-afternoon. I think I'd still want a wide zoom, 18mm in ff might be too wide at least some of the time and 24mm not wide enough, so of those two, I'd likely take the 18mm.
Without people in the scene, you might be able to get longer exposures. It's possible to find ledges, lean against the canyon walls, etc. For now, tripods aren't allowed on tours. It's very bright at the top, whatever sun/sky conditions are, and dark near the bottom. Note the light at the bottom has bounced around the brownish red walls getting there. I'd suggest shooting raw and a perhaps a color card or piece of clothing for comparison in processing. This is a 2015 shot so there's been improvement in sensors, processing, etc., since then.
You will not want to change lenses. Even if not in one of the spots were they toss sand to get light beams (Upper Canyon, certain times of the day/year), it's windy and dust blows into the canyon and sifts down pretty much constantly.